Mix the butter with the sifted sugar and vanilla sugar until frothy. Stir in the individual egg yolks, lemon juice and zest. Mix the flour, baking powder and chocolate chips and mix alternately with the egg liqueur into the yolk mass. Beat the egg whites into snow and fold into the dough. Pour the dough into a greased, floured Gugelhupf pan and bake in the preheated oven at 180 ° C for about 60 minutes. Let the Gugelhupf cool down and sprinkle with icing sugar and cocoa.