Plum Crumble Bars with Walnut Shortbread Crust (Video)
Plum crumble bars are a thick layer of jammy plums between a buttery walnut shortbread base and a craggy walnut crumble top. They cut into neat squares, and they are just as good cold from the fridge the next day. Let me show you how.
If you have Italian prune plums to get through, my plum galette with almond cream and my almond flour plum tart both put them to work, and the plum butter cookies use them once the fruit has already gone into jam.
Here is why you will love these Plum Crumble Bars
Walnuts are what set these apart from every other plum bar, and they show up twice: once in the base and once on top.
- Tart fruit on a buttery base – the plums concentrate and stay tangy while the shortbread underneath bakes like a cookie, rich and firm. The brown sugar in the crumble takes the sharp edge off without turning any of it into candy.
- The plums stay in pieces – halved and set cut side up in tight rows, they soften into pockets of fruit instead of collapsing into jam. You get whole pieces in every square.
- Pink-fleshed plums bake mellow – green-fleshed ones turn sour in the oven, and pink ones do not. I have baked both, and if you get a choice at the market, take the pink ones. They bake sweet enough on their own.
- The crust goes up the sides – pressed about an inch and a half up the pan; it gives the plum juice a wall to stop at. No soggy bottom, nothing to pour off.
- Cooling is what sets them – the cornstarch only thickens as the slab comes down to room temperature, so a day in the fridge gives you clean squares and costs you nothing.
- The same base works with any fruit – apples, pears, peaches, raspberries, blueberries, whatever is in season. The shortbread and the walnut crumble stay the same.
What Are Plum Crumble Bars?
They’re shortbread bars with fruit baked in the middle and streusel on top. All the flavor of a pie, with no rolling pin and no pie plate. The base is pressed into the pan rather than rolled, so there is no pastry to chill or fit.
This one started as a pear crisp bar I made with walnuts a couple of falls ago. When prune plums came in I swapped the fruit and left everything else alone. Halved plums hold their shape where cubed pears go soft, so you get distinct fruit in every square.
Ingredients for Plum Crumble Bars
Walnuts do double duty here, in the shortbread and in the crumble, and cornstarch decides whether the middle sets or runs. Amounts are in the recipe card at the bottom.
- Italian prune plums: cut one open before you bake – pink or amber flesh bakes mellow and sweet; green flesh bakes tart. Red or black plums work too; they are just juicier.
- Walnuts: roasted rather than raw; it deepens the flavor a lot. Raw will do in a pinch, and pecans swap in straight.
- Butter: unsalted. Soft for the crust, cold for the crumble.
- Brown sugar: light or dark, either one. The molasses is what makes the crust taste caramelly against the tart fruit.
- All-purpose flour: regular all-purpose, nothing special. It gives the shortbread its snap and holds the crumble in clumps.
- Cornstarch: this is what sets the filling. Tapioca starch works in its place.
- Vanilla and almond extract: both go into the crust. The almond is what makes the walnuts taste rounder next to the plums.
- Kosher salt: a little in the crust and a little in the crumble. It keeps the bars from tasting flatly sweet.
How to Make Plum Crumble Bars (Step-by-Step)
The crumble goes into the freezer before anything else, so start there and let it firm up while you build the rest.
- Make the crumble first. Rub or pulse the butter, brown sugar, flour, and kosher salt together until the mixture holds in chunks, then mix in the chopped walnuts. Put the bowl in the freezer.
- Cream the butter and sugar. Beat the room-temperature butter with the brown sugar until smooth.
- Finish the shortbread. Mix in the flour, vanilla, almond extract, and kosher salt until it comes together into a chunky dough, then work in the walnuts.
- Heat the oven and line the pan. Preheat to 350°F. Line a 9×9 inch pan with parchment, leaving about 2 inches up the sides. That overhang is what you lift the finished slab out by.
- Coat the plums. Toss the halved, pitted plums in cornstarch until every cut side is covered, then fold in the brown sugar and vanilla.
- Press in the base and up the sides. Spread the shortbread across the pan, press it down firmly and evenly, then bring it up the sides about an inch and a half. That wall is what holds the plum juices in until they set as the bars cool.
- Layer the plums. Arrange the halves cut side up in tight rows. Do not shake the pan to settle them – that can loosen the crust you just pressed in. Scrape any sugar and cornstarch left in the bowl over the fruit before adding the crumble.
- Add the crumble. Break the frozen crumble into chunks over the plums. If it looks sandy, squeeze handfuls together first, then break them over the top.
- Bake until the center reads 200°F. About an hour and twenty minutes at 350°F. The top goes deep golden well before the middle is set, so go by the thermometer and wait for the center to bubble. If you take it out before the center bubbles, the juices stay thin, and the cornstarch never thickens them.
- Cool, then cut. Warm from the oven, it is a spoon dessert with ice cream. For clean squares, let it cool completely, lift the slab out by the parchment, and cut it on a board.
Marina’s Pro Tips for
the Best Plum Crumble Bars
Almost everything that goes wrong with a fruit bar comes down to water, and most of what follows is about keeping it where it belongs.
- Use the pink-fleshed plums, not the green ones. Cut one open before you start. Pink or amber inside bakes mellow and sweet; green inside turns sharply tart once it is baked, because the oven drives off water and leaves the acid behind. Sort them by color, and the bars come out sweet instead of sour.
- Put the crumble in the freezer first. Room-temperature crumble melts into the fruit, and you lose the crunchy top. Twenty minutes is enough, and the clumps stay clumps.
- Squeeze it into clumps, not sand. If the crumble comes out of the bowl looking sandy, press handfuls together in your palm, then break them over the plums. Bigger pieces bake into the craggy top that makes these worth eating.
- Take the crust up the sides. About an inch and a half, all the way around. The plums give off a lot of juice, and that wall keeps it in the bars instead of letting it run under the crust.
- Roast the walnuts before adding them. Bake for 10 minutes at 350°F to make them taste like walnuts in the finished bars instead of getting lost under the brown sugar. In a pinch, raw is fine; you just do not get that depth.
- Cornstarch before sugar, not after. Sugar starts pulling juice out of the plums the second it touches them. Coating dry fruit in cornstarch first gets it on evenly, so the filling sets instead of running out the sides.
- The center has to hit 200°F. The top browns long before the cornstarch sets, so a golden top alone doesn’t mean it is ready. An instant-read thermometer in the middle of the pan tells you when it’s ready.
- Cool it completely before the first cut. Warm fruit is still liquid, so the bars slide apart under the knife. After a few hours on the counter, or an hour in the fridge, cut into squares that hold together.
Ways to Serve Plum Crumble Bars
These go both ways, warm and soft or cold and firm, and they are a different dessert each time.
- Warm with vanilla ice cream – spoon it straight from the pan while the fruit is still loose.
- Cold with coffee – a night in the fridge firms them up, and they eat more like a pastry.
- On a dessert board – cut them small and set them next to plum butter cookies for a plum table.
- Packed for lunches – they travel better than pie and do not need a plate.
How to Store Plum Crumble Bars
Room temperature: loosely covered, up to 2 days. The crumble stays crispest this way.
Fridge: in an airtight container, up to 5 days. They firm up and cut more neatly when cold.
I don’t freeze these: cornstarch and very juicy fruit weep once they thaw, so the filling comes back watery and the crust goes soft. They keep well enough in the fridge that it is not worth it.
To reheat: 10 minutes in a 325°F oven brings the crumble back. A microwave softens it instead.
Make it ahead
Only the crumble freezes. Mix it, bag it, and it keeps a month, so that part is done before you start. The shortbread can go into the lined pan, covered, up to 3 days ahead. Bake them the day you want them, since the top is crispest the first day. The baked bars do not freeze, for the reason in storage above.
Below are the questions that come up most often about baking plum bars.
Plum Crumble Bars FAQ
Why are my Plum Crumble Bars runny in the middle?
Almost always because they came out of the oven early. The center needs to reach 200°F for the cornstarch to set, and the top browns long before that happens. Another cause is juicier fruit: round red or black plums carry more water than Italian prune plums, so drain them in a sieve for 10 minutes after mixing with the sugar.
Why did my Plum Crumble Bars turn out sour?
Almost always the plums, not the recipe. Cut one open before you bake. Plums with pink or amber flesh bake mellow and sweet. Plums with green flesh are much higher in acid, and baking makes that worse rather than better, because the water cooks off and the acid stays behind.
Both can be perfectly ripe and still behave this way, so ripeness is not the thing to go by. If green-fleshed plums are all you have, add a little more sugar to the fruit.
Can I use regular plums instead of Italian Prune Plums?
Yes. Cut round plums into quarters rather than halves, since they are bigger, and let them sit in a sieve for 10 minutes after you add the sugar. The bake time stays the same, and so does the 200°F cue.
Can I use frozen plums?
Yes, and they do not need thawing. Toss them in the cornstarch while still frozen, add the sugar and vanilla, and add about 10 minutes to the bake. Thawed plums dump their juice into the bowl, and the filling never quite recovers.
Frozen fruit weeps more, so there is more juice to thicken. Cool the bars completely before cutting, or they will not hold a square.
Can I make Plum Crumble Bars in a 9×13 pan?
The recipe is written for a 9″ x 9″ pan, which is the one in the photos. A 9×13 works, but the bars come out thinner, so start checking the center at 1 hour 10 minutes. An 8-inch square pan works too with two-thirds of the recipe, or a full batch baked deeper for about 1 hour 35 minutes.
Can I swap the walnuts for another nut?
Pecans are the closest match and need no other changes. Hazelnuts and almonds both work and both taste sweeter, which shifts the balance against the fruit. For a nut-free version, leave them out of the crust and crumble, and add rolled oats to the crumble instead.
More Plum Recipes to Try
Prune plum season is short, so here is the rest of what I do with them while they last.
Plum Galette with Almond Cream
A free-form tart with almond cream under the fruit. Same plums, one pastry round, no pan.
Plum Butter Cookies (Pesochnoe Pirozhnoe)
Shortbread sandwiches filled with plum butter and buttercream. What to bake once the plums are already jam.
Plum Butter Pirog
Tart dough, plum butter, roasted walnuts, and meringue. The closest cousin to these bars.
Watch Marina Make These Plum Crumble Bars
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Plum Crumble Bars with Walnut Shortbread Crust
Ingredients
Walnut Crumble
- 2 oz unsalted butter Cold
- 1/2 cup brown sugar Packed
- 1/2 cup all-purpose flour
- 1 cup walnuts Roasted and chopped
- 1/4 tsp kosher salt
Walnut Shortbread Crust
- 4 oz unsalted butter Room temperature
- 1/2 cup brown sugar Packed
- 1 1/4 cups all-purpose flour
- 1 tsp vanilla extract
- 1 tsp almond extract
- 3/4 cup walnuts Roasted and chopped. Pecans work too.
- 1/4 tsp kosher salt
Plum Filling
- 2 lb Italian prune plums About 24. Halved and pitted. Pink or amber flesh bakes sweeter than green.
- 1/2 cup cornstarch
- 1 cup brown sugar Packed
- 1 Tbsp vanilla extract
Instructions
Make the Walnut Crumble
-
Mix the crumble.
In a food processor: pulse 2 oz unsalted butter, 1/2 cup brown sugar, 1/2 cup all-purpose flour and 1/4 tsp kosher salt until it clumps.
By hand: rub the same four together with your fingertips. Either way, stop when it holds together in chunks, then mix in 1 cup walnuts, roasted and chopped. -
Freeze it. Put the bowl in the freezer and leave it there until you assemble. Cold crumble keeps its clumps in the oven.
Make the Walnut Shortbread Crust
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Cream the butter and sugar. Beat 4 oz unsalted butter, at room temperature, with 1/2 cup brown sugar until smooth.
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Add the flour and extracts. Mix in 1 1/4 cups all-purpose flour, 1 tsp vanilla extract, 1 tsp almond extract and 1/4 tsp kosher salt until it comes together into a chunky dough. Mix in 3/4 cup walnuts, roasted and chopped, and set it aside.
Prep the Pan and the Plums
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Heat the oven and line the pan. Preheat to 350°F. Line an 9x9 inch pan with parchment that comes about 2 inches up the sides, so the whole slab lifts out later.
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Coat the plums in cornstarch. Halve and pit 2 lb Italian prune plums, then toss them with 1/2 cup cornstarch until every cut side is coated.
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Add the sugar and vanilla. Fold in 1 cup brown sugar and 1 Tbsp vanilla extract. Do this right before you assemble so the sugar has no time to pull the juice out.
Assemble and Bake
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Press in the base, and up the sides. Spread the shortbread over the bottom of the lined pan, press it down firmly and evenly, then bring it up the sides about 1 1/2 inches. The flat bottom of a measuring cup does this well. That wall holds the plum juices in while the bars set.
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Layer the plums. Arrange the halves over the crust cut side up, packed tight. Do not shake the pan to settle them - it can loosen the crust you just pressed in. Scrape whatever sugar and cornstarch is left in the bowl out over the fruit.
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Top with the frozen crumble. Break it into chunks over the plums. If it looks like sand, press handfuls together in your palm first, then break those over the top.
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Bake about 1 hour 20 minutes. The top should be deep golden, the center should be bubbling, and a thermometer in the middle should read 200°F to 205°F. If you take it out before the center bubbles, the juices stay thin and the cornstarch never thickens them.
-
Cool before cutting. Warm from the oven it is a spoon-and-ice-cream dessert. For clean squares, cool it completely, lift the slab out by the parchment and cut it on a board.
Recipe Notes
- Pan: a 9" x 9" pan, the one in the photos. Other pans work. A wider pan spreads the bars thinner and they bake faster, so follow the temperature cue below rather than the clock.
- Plums - check the flesh color: cut one open before you bake. Pink or amber flesh bakes mellow and sweet. Green flesh bakes tart, because the oven drives off water and concentrates the acid. Both can be fully ripe, so go by color, not ripeness. With green-fleshed plums, add a little extra sugar to the fruit.
- Plum variety: Italian prune plums (small, oval, deep blue) hold their shape and are not too wet. Regular red or black plums work; cut them into quarters and let them drain in a sieve for 10 minutes after mixing with the sugar.
- Doneness: 200°F to 205°F in the center is the cue that matters. The cornstarch needs that heat to set, and the top browns before the middle gets there.
- Yield: about 4 lb (1.85 kg) baked. Cut 3 across and 4 down for 12 bars of roughly 5.4 oz (155 g) each.
- Make it ahead: the crumble keeps in the freezer for up to a month, so you can make it well in advance.
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