Mix the custard powder and sugar and mix with 4 tablespoons of milk. Bring the rest of the milk to the boil, remove from the stove and stir in the pudding mixture quickly and vigorously. Bring to the boil, stirring constantly, remove from heat and let cool down.
Stir the quark into the slightly cooled - but not yet firm - pudding. Whip the whipped cream with the vanilla sugar until very stiff and fold it into the cool cream. Serve cold.