Mix the butter, sugar and vanilla sugar until frothy. Gradually stir in the eggs well one by one. Mix and stir in cinnamon, nuts, flour and baking powder. Finally, briefly fold in the chopped chocolate with the sour cream.
Pour the dough into a greased springform pan (26 cm diameter) and smooth it out.
Halve the washed plums, core them and distribute evenly on the dough with the cut surface facing up.
Bake in the oven at 180 degrees for about 45-50 minutes.
Dust them with a little icing sugar when they have cooled down.