Mix the butter, sugar, vanilla sugar and egg yolk until creamy. Mix the flour with the baking powder and sieve to it. Add milk and stir everything together. Beat the egg whites with a pinch of salt and carefully fold them into the dough. Finally, divide the dough into two portions and mix one half with the cocoa and Nutella. Possibly stir in a spoon or two of milk.
Then alternately pour the dough into a greased and crumbled bundt cake pan.
As with the marble cake, pull through with a fork at the end.
The baking time is about 50-60 minutes at 175 degrees.
Then do a chopstick test.
If necessary, cover the cake while baking so that it does not get too dark on top.