Mix eggs, milk, oil, sugar and cocoa together to make a liquid sauce. Set aside half a drinking glass of this mixture.
Add flour, baking powder and vanilla sugar to the rest of the liquid, mix, transfer to a baking sheet or springform pan. In the case of a springform pan, extend the baking time and do a stick test.
Bake the sheet cake for about 30 minutes. remove, spread the mixture that has been set aside over it, allow to cool.