Wash the plums, drain, halve and stone them. Cream fat and 200 g sugar. Add lemon zest, stir in the eggs one by one. Mix flour, cornstarch and baking powder and stir briefly into the mixture.
Grease the fat pan of the oven (32x39 cm) and dust with flour. Put the dough into the fat pan and spread evenly. Spread plums on top. Mix egg yolk, sour cream, 50 g sugar and vanillin sugar.
Pour evenly over the fruit topping. Bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for approx. 35 minutes. Let them cool down. Results in about 20 pieces.