Put the flour, butter, sugar, egg, pinch of salt and lemon zest in a mixing bowl and work into a shortcrust pastry. Shape the dough into a ball and put it in the fridge for about 30 minutes.

Clean the rhubarb and cut into pieces approx. 2 cm in size. Put the pieces in a bowl, sprinkle with the 2 tablespoons of sugar and set aside.

Cover the bottom of a 26 cm spring form with the cooled dough and pull up an approx. 3 cm high edge. Now place the spring form in the (convection) oven preheated to 180 degrees and blindly bake for approx. 10-15 minutes.

Beat the softened butter, sugar, vanilla sugar and eggs until frothy. Stir in the quark and mascarpone. Mix the cornstarch with the baking powder and stir into the mixture. Add the rhubarb pieces and fold in.

Place the mixture on the pre-baked base and bake again at 180 degrees for approx. 55 - 60 minutes. If the covering becomes too dark, cover with aluminum foil for the last 15 minutes. When the baking time is over, take it out of the oven and let it cool down in the tin for 15 minutes. Carefully remove the edge, place on a cake rack and let cool down.