Mix the butter in pieces, icing sugar, flour, 1 pinch of salt and vanillin sugar with the dough hooks of the hand mixer to form crumbles. Put about 2/3 of the crumbles into a greased rectangular tart tin with lift-off base (approx. 11 x 36 cm). Press down to a smooth base, pressing up at the edges, chill the remaining crumbles
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 15 minutes, remove from oven. In the meantime clean, wash and cut the rhubarb into small pieces. Boil up apple juice and 25 g sugar in a pot. Add the rhubarb, simmer for about 1 minute. Mix starch with 1 tbsp. water, add while stirring, let simmer for another 1-2 minutes, remove from heat
Separate the egg. Mix quark, egg yolk, 3 tbsp. sugar and lemon rind. Stir in sauce powder. Beat the egg white and 1 pinch of salt with the whisk of the hand mixer until stiff, fold in
First spread the rhubarb compote on the base, then add the cheese mixture. Sprinkle the rest of the crumbles over it. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 35-40 minutes on the bottom shelf, remove from oven, and let cool in the mould on a cake rack for about 2 hours. Remove the cake from the tin, cut into 12 bars
waiting time approx. 2 hours