For the marble-oil cake, whisk the yolks, sugar and vanilla sugar until fluffy. Add oil and water and fold in flour, baking powder and beaten egg whites from 4 egg whites.
Take away about 1/3 of the dough and stir in the cocoa.
Fill the whole dough into a greased and floured cake pan and bake the marble-oil cake at 180° C for about 1 hour.