Mix sugar, vanilla sugar and butter and stir until frothy. Gradually stir in the eggs until you get a smooth mixture. Mix a mixture of sour cream and Nutella in an extra bowl and fold into the dough. Then briefly stir in the flour and baking powder until a smooth dough is formed.
Grease a pan well and dust it with a little flour. It doesn`t matter what shape you take. I mostly use a loaf pan. Just make sure that the dough rises relatively high, but collapses a little when it cools down.
Bake for about 90 minutes and ideally let it cool in the pan, as it is very soft as long as it is still warm.