Whisk the egg yolks with sugar into the foam, add 3 tablespoons of water, oil, flour mixed with corn starch and baking powder. Finally add the cocoa and mix. Whisk the egg whites to stiff froth and then mix them into the yolk mass.
Put the prepared dough on a baking tray, lined with paper and bake in a pre-heated oven at 350 °F/ 175 °C for about 20 - 25 minutes.
Once baked, take it out of the oven and cut in half. Remove one half from the baking tray and leave the other half on the baking tray. Pierce the cake on the baking tray with a fork, pour over the prepared coffee and let the cake absorb it.
Prepare the cream. Add 2 tablespoons of common wheat flour to 3/4 cup of milk. Mix properly to avoid lumps, add 4 egg yolks and 4 tablespoons of sugar together with 1 pack of vanilla sugar. Cook on a low heat until the mass thickens. Remove from heat and let it cool. Add the butter to the slightly cooled mixture and melt it while stirring.
Prepare the white cream. Heat the water together with sugar. Once the sugar is melted, remove from heat and let it cool. Beat the egg whites into a solid foam and gradually add warm water to the egg whites and whisk until solid.
Lay the butter filling on the dry half of cake, place the wet half of cake on it, layer the white filling and pour on top the melted chocolate mixed with oil. Refrigerate and serve. Enjoy!