Mix the butter with sugar, vanilla sugar and salt. Gradually stir in the eggs. Mix the flour with baking powder and mix in alternately with the milk. Pour half of the dough into a small (diameter approx. 19 cm) greased bundt cake pan. Stir in the cocoa powder under the other half and spread this batter on the light-colored batter.
Bake in the preheated oven at 175 ° C top / bottom heat for approx. 45-50 minutes.