Kartoshka Rum Balls are made with a delicious sponge cake, sweetened condensed milk buttercream, roasted walnuts, and cocoa to resemble a potato, which is what these pastries were named after.
Whip 6 eggs with a mixer on high speed, in a grease-free bowl until frothy. Gradually add the sugar, pouring it between the bowl and the whisk. Continue whipping for 15 minutes until pale in color, thick and flows off the whisk in a ribbon.
While the eggs are whipping, turn the oven to 350F. Also, line a 9"x13" baking pan with foil and spray with nonstick spray, or butter the pan and add a heaping spoon of flour, then shake to coat it with flour. Shake out the excess.
Sift the flour and baking soda mixture into the egg mixture, in thirds, carefully folding it in to prevent deflating the batter.
Add room temperature butter to the mixer and whip with on high until white in color and fluffy, about 4 minutes.
While continuing to whip, add the sweetened condensed milk by allowing it to flow down the inside of the bowl. If the buttercream starts to look broken, stop the mixer right away and fold the remainder of the sweetened condensed milk in with the spatula.
Break the cooled cake into small pieces. Add the broken pieces of cake and rum to the buttercream (and cocoa powder, if using) and using the beater attachment combine together, for about 2-3 minutes or until it starts looking like dough. Add the nuts and mix until combined. Taste the mixture and add more rum if desired.
Pinch off about 1.5 -2 tablespoons of the cake ball mixture and form a ball. Repeat with the rest of the mixture. If the mixture is too soft to roll, refrigerate until it's malleable and easy to roll.
Combine 3 tablespoons cocoa powder and 3 tablespoons powdered sugar. Stir to combine. Dip and roll each ball in the mixture. Set aside. Once done, coat each ball in the cocoa mixture one more time.
Alternatively, you can melt 2 cups of dark chocolate melts over a double boiler, then roll each rolled cake ball in the chocolate and set them on parchment paper to set. This way the chocolate will dry into a hard coating.
Or make 2 recipes of this Extra Shiny Chocolate Ganache and coat each cake truffle in it. The chocolate pastries will be a tad sticky, since the ganache does not dry into a hard coating. Finish by piping the "potato sprouts" on top of the Kartoshka with the reserved buttercream.
The base of Kartoshka cake truffle is cake crumbs. You can either make one from scratch like I show in this recipe, or replace it with 4 cups of crushed mild flavored cookies like Nilla wafers or even Graham Crackers.