Mix the custard base. In a saucepan, whisk 3 egg yolks with 2/3 cups granulated sugar. Slowly whisk in 1 cup whole milk until the sugar dissolves. Add 4 Tbsp cocoa powder and mix until no cocoa streaks remain. Lumpy cocoa? Run the mixture through a sieve.

Cook the custard. Set over medium heat and cook, whisking nonstop so the custard does not scorch, until it thickens and coats the back of a spoon.

Do not boil - the eggs will curdle and separate.

Doneness test: dip the spoon and run your finger across the back. If the streak stays clean, the custard is done. Remove from heat and cool completely.

Whip the butter. With a mixer, whip 3/4 cup unsalted butter at room temperature for about 5 minutes, until lighter in color and fluffy. Scrape the sides of the bowl several times, so no butter is left unwhipped.

Add 20 oz sweetened condensed milk or dulce de leche in two additions, whipping well after each one.

Add the cooled custard in two additions, whipping thoroughly in between. Set aside until needed.

Make the dough. Whip 7 Tbsp unsalted butter for about a minute. Add 2 large eggs and mix well. Mix 1 1/2 cups sour cream with 1 tsp baking soda, then add to the butter mixture. Add 3 - 3 1/2 cups all-purpose flour in two additions, mixing until a dough forms. Do not overwork the dough.

Divide the dough. Split it into 13-14 pieces, about 80 g each, and roll into balls. Keep them covered with plastic wrap while you work.

Roll and bake. Flip a jelly roll sheet upside down. Roll two balls of dough directly on the back of the sheet into circles a bit bigger than an 8-inch round pan. No flour needed.

Bake at 400°F until slightly golden, about 5-7 minutes. Repeat with the rest of the dough.

Trim while warm. As soon as a layer is baked, set the 8 inch round pan on top and trim right around it. Crush the trimmings and save them for the sides, or freeze them in a ziplock for another cake.

Stack and frost. Put a dab of frosting on a plate and set the first layer on top. Add a generous spoonful of frosting, about 75 g, and spread it into an even layer.

Reserve about 1/2 cup of frosting for covering the cake at the end.

Repeat. Keep baking, trimming, and stacking with frosting until all the layers are done.

Cover and press. Cover the sides and top with the reserved frosting. Set a cake ring around the cake if you have one - it keeps the sides straight. No ring is fine too. Cover with plastic wrap, set a cutting board on top, and weigh it down with something heavy. Let it sit at room temperature for up to 24 hours.

Whip the chocolate cream. The next day, whip 1 1/2 cups heavy whipping cream with 1/3 - 1/2 cups powdered sugar and 1-2 Tbsp cocoa powder until fluffy.

Decorate. Cover the cake in a thin layer of the whipped cream, smoothing the top. Fit a ziplock bag with a large star tip and fill it with the rest. Pipe stars along the border. Set a peeled hazelnut in the middle of each star. Dust with cocoa if you like.

Serve at room temperature. That is when the layers are softest.