Mix the butter at room temperature until it becomes frothy and then add the powdered sugar and mix until incorporated.
Separately beat the egg whites with a pinch of salt. Add the foam over the butter and mix gently, with movements from bottom to top.
Mix ground walnuts with flour, baking powder, vanilla and cocoa then add this mixture over the butter foam and mix gently until incorporated.
Put the dough in the tray lined with baking paper and cook for 20 minutes.
Meanwhile, mix the yolks and powdered sugar well until they turn white and double in volume. Remove the tray from the oven and spread the yolks over the entire surface of the cake.
Bake for another 10 minutes.
Remove the cake from the pan and let it cool.
For decoration, melt the chocolate with 2-3 tablespoons of milk. With a spoon draw lines over the yolk icing.
After the chocolate has hardened, we can portion it.