Cut butter into pieces. Knead butter, 50 g sugar, egg yolk and flour with the dough hooks of the hand mixer. Then work into a smooth dough with your hands. Cover and chill for about 30 minutes. Grease the tart tin (20 x 28 cm). Roll out the dough rectangularly (approx. 22 x 30 cm) and line the tin with it. Whip the protruding ends inwards and press firmly. Prick the dough several times with a fork.
Place baking paper on the dough, fill in dry peas. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 25 minutes, remove baking paper and dry peas after approx. 20 minutes. Remove the base from the oven and let it cool down. Wash 400 g rhubarb, clean and cut into pieces. Caramelise 2 tablespoons of sugar. Add rhubarb and 1 teaspoon raspberry syrup and cook covered for about 5 minutes. Puree the rhubarb and let it cool down a little. Soak 1 1/2 and 3 1/2 leaves of gelatine separately in cold water. Sort and puree the blackberries. Squeeze 3 1/2 leaves of gelatine and stir into the warm rhubarb puree. Squeeze lemon. Mix lemon juice, mascarpone, sour cream, 50 g sugar and vanillin sugar. Divide the mixture into three. Squeeze 1 1/2 sheets of gelatine and dissolve in a small pot. Stir in the blackberry puree, then stir into 1/3 cream. Chill the cream until it starts to gel.
Sort and puree the blackberries. Squeeze 3 1/2 leaves of gelatine and stir into the warm rhubarb puree. Squeeze lemon. Mix lemon juice, mascarpone, sour cream, 50 g sugar and vanillin sugar. Divide the mixture into three. Squeeze 1 1/2 sheets of gelatine and dissolve in a small pot. Stir in the blackberry puree, then stir into 1/3 cream. Chill the cream until it starts to gel. Stir the rhubarb puree into 2/3 of the cream and chill until it starts to gel. Wash, clean and slice 100 g rhubarb. Boil 300 ml water, 50 g sugar and 1 tablespoon raspberry syrup. Cook the rhubarb pieces in it for 1-2 minutes, then take them out immediately and put them in a cool place. Whip the cream until stiff. Fold 1/3 into the blackberry cream, 2/3 into the rhubarb cream. Finally fold in the rhubarb pieces. Fill the creams alternately marbled into the tart tin and chill for at least 4 hours
Stir the rhubarb puree into 2/3 of the cream and chill until it starts to gel. Wash, clean and slice 100 g rhubarb. Boil 300 ml water, 50 g sugar and 1 tablespoon raspberry syrup. Cook the rhubarb pieces in it for 1-2 minutes, then take them out immediately and put them in a cool place. Whip the cream until stiff. Fold 1/3 into the blackberry cream, 2/3 into the rhubarb cream. Finally fold in the rhubarb pieces. Fill the creams alternately marbled into the tart tin and chill for at least 4 hours