Mix the butter with the sifted sugar, vanilla sugar, yolk, rum and salt until frothy. Add the softened chocolate to the yolk. Mix flour with baking powder, sieve and mix alternately with milk into the dough. Beat egg whites into snow and fold in. Pour the dough into a greased, floured Gugelhupf pan and bake in the preheated oven at 180 ° C for about 50 minutes.
Let the Gugelhupf cool down and decorate with chocolate glaze as desired.