Grease and flour a bundt pan. Preheat the oven to 180 ° C.
Mix the butter with the sifted sugar, gradually the egg yolks, cinnamon and rum until frothy. Beat the egg whites with vanilla sugar to a snow. Melt the chocolate over the water bath and let cool down a little. Stir into the butter mixture. Mix the flour and baking powder and sieve in alternately with the buttermilk. Fold in the egg whites.
Bake the Gugelhupf for about 50 minutes. Decorate as desired or cover with chocolate icing.