Separate the eggs. Beat yolks (egg yolks) with half of the sugar and vanilla sugar until very foamy. Mix chestnut puree, flour and hazelnuts. Grease a cake pan with butter and dust with flour. Beat the egg whites with the remaining sugar until stiff. Mix the yolk mixture with some of the beaten egg whites and fold in the flour and chestnut mixture and the remaining beaten egg whites. Finally, add the melted butter. Pour the mixture into the gugelhupfform. Bake in the preheated oven at 160 – 170 °C for about 45 minutes. Let stand briefly, then turn out and sprinkle the cooled Gugelhupf with powdered sugar.