Cut the vanilla pod lengthwise and scrape out the pulp. Mix flour, 175 g sugar, vanilla pulp and salt in a bowl. Add 1 egg and butter and knead into crumbles first with the dough hooks of the hand mixer, then with your hands. Press just under 2/3 of the dough (approx. 400 g) as a base into a greased springform pan (28 cm Ø) sprinkled with flour. Place the mould in a cool place for 10-15 minutes.
Mix mascarpone, quark, 100 g sugar, 6 eggs, lemon juice and lemon peel. Sift the starch over it and stir it in. Spread plum puree on the crumble base. Spread the mascarpone cream over it and smooth it down. Bake in the preheated oven, lower shelf (electric oven: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 35-40 minutes
Wash the plums, drain well, cut in half and remove the stones. Halve the halves again lengthwise. Remove the cake from the oven. Place the sliced plums on the cake in a circle, with the cut surface facing upwards. Spread the rest of the crumbles on top and bake the cake at the same temperature, but on the middle shelf, for 30-35 minutes
Remove the finished cake from the edge with a knife and let it cool in the mould on a cake rack