Stir QimiQ until smooth. Add the low-fat curd, sugar, vanilla sugar and lemon juice and mix well.
If you have chosen fruit, fold it in at this time. Fold in whipped cream.
Cut the sponge cake layer apart in the middle. Place the bottom half in the cake ring, spread with the cream, place the other bottom on top. Allow the cake to set in the refrigerator for a few hours. Dust with powdered sugar just before serving.