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Beat the butter, icing sugar and vanilla sugar and a pinch of salt until fluffy. Gradually stir in the egg yolks.Whisk the egg whites with the sugar until stiff and fold into the egg yolk mixture. Mix in the flour. Color about a third of the mixture with cocoa.
Grease the gugelhupf basin with butter, and dust with flour. Now fill the form alternately with the light and dark mixture. Bake in a preheated oven at 180 °C for about one hour. Overturn the cake onto a cake rack and allow to cool. Sprinkle with icing sugar.