For the compote, wash, stone and chop the plums. Bring to the boil with about 100 ml nectar and 1 tbsp. sugar. Cover and simmer for about 3 minutes. Mix starch with the rest of the nectar until smooth. Bind plums with it. Let the compote cool down.
Separate the eggs for the sponge cake. Beat the egg whites, 1 pinch of salt and 2 tablespoons of cold water until stiff. Let 100 g sugar trickle in. Stir in egg yolk. Sift flour and baking powder over the egg yolk and fold in. Spread into a springform pan (26 cm Ø) lined with baking paper. Bake in a hot oven (electric cooker: 175 °C/circulating air: 160 °C/gas: see manufacturer) for about 20 minutes. Let it cool down.
For the cream, soak gelatine cold. Mix quark, mascarpone, vanilla sugar and 125 g sugar. Dissolve the gelatine carefully. First stir in 5 tbsp. cream, then stir into the rest of the cream. Beat 250 g cream until stiff and fold in.
Cut the sponge in half crosswise. Place a cake ring around the lower cake base. Spread half the cream on the bottom. Put half of the compote in blobs on top and "swirl" it with a fork. Place the upper cake base on top, spread the rest of the cream and the rest of the compote on top. Chill for at least 4 hours.
Whip 150 g cream until stiff. Remove cake from the ring. Spread the cake rim with cream.