Beat eggs, sugar, mineral water, then fold in flour and 1-2 tablespoons cocoa powder.
Pour the mixture onto a greased baking tray (be careful!!! not too thick) and bake at 200 °C for about 15 minutes. Cut the still warm cake base in half. (or bake double mass)
For the filling of the milk slice, cook a pudding from milk, 2-3 tablespoons sugar and the cornstarch. Allow to cool. Then fold in the whipped cream. Dissolve the soaked gelatine and finally stir into the mixture.
Spread the cake base evenly with the milk cream, then place the lid on the mixture and cut into even pieces.