For the marble meringue cake, cream the butter together with the sugar, salt and vanilla sugar, separate the eggs and add the yolks one by one, stirring them in gradually.
Cream the entire mixture very well with a hand mixer or food processor. Sift the flour with the baking powder and stir into the batter alternately with the rum.
Beat the egg whites and carefully stir them into the dough. Now divide the dough into 2 halves and stir the cocoa powder into one half.
Fill the dark and the light mixture alternately into a well greased and floured cake pan and bake the cake in a preheated oven (180°) for 50 – 60 minutes. Test with a needle.
Before serving, sprinkle the Gugelhupf with powdered sugar.