Mix 150 ml of lukewarm water and dry yeast (included in the package) in a bowl. Add the baking mix and 1 egg and knead with the dough hooks of the hand mixer on the highest setting for approx. 3 minutes.
Cover and leave to rise in a warm place for 20-30 minutes. In the meantime quarter plums. Peel and quarter apples and cut out the core. Cut the apple quarters into cubes and sprinkle with 3-4 tablespoons of lemon juice.
Roll out the yeast dough on a greased fat pan of the oven (33x37cm) and let it rise again for about 20 minutes. Separate remaining eggs for the glaze. Mix egg yolk, quark, mascarpone, 75 g sugar, vanillin sugar, lemon peel and cornstarch.
Beat the egg whites until stiff, while allowing the remaining sugar to trickle in. Carefully fold into the icing. Sprinkle apples and plums on the dough and pour the icing over it. Smooth with a dough scraper.
Bake in the preheated oven (electric oven: 175 °C/ gas: level 2) for approx. 40 minutes. Let the cake cool down and sprinkle with icing sugar and cinnamon. Cut into approx. 24 pieces and arrange on a plate decorated with lemon balm as desired.