To prepare Gerbeaud cake, grind the walnuts, mix them together with powdered sugar, add lemon zest, then set the mixture aside.
Heat up the milk to a lukewarm temperature, add yeast and a tablespoon of granulated sugar. Set it aside for 15 minutes, which gives you enough time to measure out and mix your other ingredients.
Combine flour with powdered sugar, then add butter. Work the mixture with your hands until you achieve a slightly crumbly texture.
Add two egg yolks, a pinch of baking soda and the yeast mixture.
Knead the dough until it doesn’t stick to your hands. (Approx. 5 mins)
Divide the dough into 3 equal parts. Lightly dust your working surface with flour, and start rolling out the first piece of dough. Spread butter and dust flour onto a 35×40 cm (13x15 inch) baking pan (you can also use parchment paper). Transfer the thinned-out dough onto a rolling pin, then lift it into the pan. Make sure you cut the dough to size with a knife, so it fits the pan. Cut off the overhanging parts to fill up any gaps.
Spread a generous amount of apricot jam (approx. 4-5 heaping tablespoons of jam per layer), followed by a layer of the lemon zest and walnut mixture.
Onto the second layer! Lay down the next layer of dough, spread the other half of the jar onto it, and sprinkle the remaining walnuts over it. Finally, stretch out the third dough portion, which will form the top layer of the cake.
Pierce the top layer a few times with a fork (make sure your fork goes through all layers). Cover the cake with a clean kitchen cloth and let it rest for 1 hour.
After an hour, preheat your oven at 175 °C (375 F) and bake the cake for 35 mins until the top layer gets a lovely golden-brown color.
Wait until your Gerbeaud cake cools down, then start preparing the chocolate glaze. In order to prepare the glaze, mix together 6 tablespoons of powdered sugar with 6 tablespoons of cocoa powder. Combine this with boiling water and a teaspoon of oil – this will make your glaze smooth and shiny. Mix it with a hand whisk until you achieve a smooth texture. Pour the glaze on top of the cake, and spread it out with a spoon or spatula. Allow the glaze to set overnight, or at least for a few hours before serving.
The following day your Gerbeaud cake is finally ready to be sliced! Only slice up as much as you think you will eat, so the cake doesn’t dry out. Cut the cake into thin rectangles or diamond shapes.