Beat butter, sugar, vanilla sugar and rum until frothy. Stir in the egg yolks one by one and continue beating. Mix the flour and baking powder and stir into the dough alternately with the buttermilk. Beat the egg whites into snow and mix in.
Grease and flour the bundt pan.
Pour a third of the batter into the mold and spread some blackberry purée on top. Repeat the process, finish with the batter.
Preheat the oven to 180 ° C and bake the Gugelhupf for about 60 minutes. Let the Gugelhupf cool down and turn it out.
Decorate with white chocolate icing, chocolate sprinkles and blackberries.