Wash the plums, drain and cut in half. Remove stones. Pour cream into a bowl, wash 1 cup and dry. Add 1 cup (200 g) of sugar, vanilla sugar and lemon zest to the cream and whisk half stiff with the whisks of the hand mixer.
Stir in 5 eggs one after the other. Sift flour and baking powder and stir into the cream-egg mixture. Spread the dough into a greased fat pan of the oven. Spread plums loosely on top and sprinkle with 1/4 cup (50 g) of sugar.
Stir the sour cream with the remaining sugar, eggs and orange peel until smooth. Spread the sour cream mixture between the plums. Bake in the preheated oven (electric: 200 °C/ gas: level 3) for about 30 minutes.
If necessary, partially cover with aluminium foil towards the end of the baking time. Results in about 20 pieces. Sprinkle with sugar crystals as desired.