For the marble cake, separate the eggs. Beat the yolks with 200 g sugar and the vanilla sugar until foamy. Slowly add oil and water and continue to beat.
Mix flour and baking powder and sift over the yolk-sugar mixture. Beat the egg whites with 20 g sugar until stiff and fold in.
Divide the dough into 2 portions. Mix the cocoa with 2 tablespoons of water and stir into one of the mixtures. Grease 2 loaf pans with butter and dust with flour.
Fill in the light and dark mixture alternately. Bake the marble cake at 180 °C for about 60 minutes.