Warm the milk lukewarm. Melt the fat in it. Mix 50 ml milk-fat mixture and yeast. Mix 25 g sugar, 1 pinch of salt and flour in a bowl and make a depression in the middle. Pour yeast milk into the middle and mix with some flour from the rim. Cover with flour and leave to rise for about 15 minutes until the surface gets cracked
Add the remaining milk and 1 egg and knead to a smooth dough. Cover the dough and let it rise in a warm place for about 45 minutes
Wash the plums, halve them and remove stones. Separate 3 eggs. Whisk the egg yolks, 75 g sugar and vanillin sugar with the whisk of the hand mixer for about 3 minutes until creamy. Mix sour cream and starch and fold into the egg mixture. Roast oat flakes and 20 g sugar in a pan for 4-5 minutes. Take out and let cool down
Grease a springform pan (26 cm Ø). Roll out the dough round on a floured work surface (approx. 25 cm Ø). Place the dough in the form, press down evenly with your hands, pressing the rim up about 1 cm. Sprinkle the dough base with breadcrumbs. Spread plums on the dough with the cut surface facing down and pour the sour cream on top. Bake the cake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 35 minutes
In the meantime, beat the egg whites until stiff, add 1 pinch of salt and 150 g sugar. Take the cake out of the oven, add the meringue, spread loosely, leaving the edges free. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for another 10-15 minutes. Remove from the oven and allow to cool on a cake rack. Decorate the cake with oat flakes and arrange on a cake plate.