Preheat the oven to 355 °F / 180 °C and prepare a 12 x 12.5 inch baking sheet, which you need to grease with oil and sprinkle with flour.
Mix the flour with cocoa and sift through a sieve. Beat the egg whites until stiff and gradually add the crystal sugar while constantly mixing and then add water. Then add the egg yolks and finally the cocoa and flour.
Layer the prepared dough on a baking sheet and let it bake. Bake for about half an hour.
Pour the prepared mixture of milk, crystal and vanilla sugar over the baked dough.
Heat up the milk. In a small bowl, whisk the egg yolks with sugar, add a little bit of warm milk and then pour this mass into the hot milk. Add flour and cook until the cream is thick, stir constantly, and let it cool.
Beat the butter with the sugar until foamy, add the cooled cream spoon by spoon. Layer the cream on the cooled base of the cake and smooth it nicely.
Top: Put 1/2 cup of sugar in a saucepan, add water (2-3 spoons) and cook on a low heat - you will create a syrup.
Beat the egg whites with the remaining sugar (1/2 cup) until stiff in a water bath and pour a thin stream of it in the hot syrup while constantly mixing. This gives the stiff egg white mixture a beautiful shine. Finally, add lemon juice, water and beat briefly.
Layer the egg white mixture on top of the yolk cream and decorate according to your own imagination - with grated chocolate, cocoa,... Leave to cool in the refrigerator before serving. Enjoy your cake!