Beat yolks with water, add sugar, vanilla sugar. Whip the egg whites and add them to the egg yolk. Add the flour with baking powder and cocoa. Bake 25 minutes at 180 degrees. Cut the cooled cake in half.
Mix mascarpone whit Nutella. Spread 3/4 of the cream on one part of the cake and place the second half on it. Spread 1/4 of the rest of the top of the cake. At the end you can decorate it with chocolate pieces and sweet cream.