Beat the butter, sugar and egg yolks very frothy. Stir in rum, chocolate, milk, nuts and baking powder. Add the flour. Beat the egg whites into snow and fold in carefully.
Fill the dough into a greased and floured Gugelhupf tin (I always use a Gugelhupf tin made of silicone, which eliminates the annoying greasing) and bake at 180 ° C (hot air) for about 50 - 60 minutes. Sprinkle with powdered sugar.