Warm milk for the yeast dough. Mix flour, yeast, 3 tablespoons sugar and 1 pinch of salt. Add 50 g butter and 1 egg. Stir in the milk with the dough hooks of the mixer. Work the yeast dough into a smooth dough for about 8 minutes.
Cover and leave to rise for about 45 minutes in a warm place.
For the topping, wash, halve and stone the plums. Separate 8 eggs. Beat the egg whites with 1 pinch of salt until stiff. Mix egg yolks with quark, cream cheese, 250 g sugar, vanilla sugar, 1 pinch of salt, lemon juice and zest with the whisks of the mixer until smooth.
Fold in 3 portions of beaten egg white.
Line a fat pan (approx. 32 x 39 cm; deep baking tray) with baking paper. Knead the dough briefly and roll out on the baking tray. Press into the corners with your hands. Prick several times with a fork.
Mix 3 tablespoons sugar and cinnamon, 2 1⁄2 Sprinkle cinnamon sugar on the dough. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
Spread the cheese mixture on the dough. Add plums and sprinkle with the remaining cinnamon sugar. Bake in a hot oven for about 35 minutes. Let the cake rest in the switched off oven with the door open for about 30 minutes.