Fill butter cookies into a freezer bag and crumble them finely with a rolling pin. Melt butter in a pot. Mix crumbs and butter. Spread them on the bottom of a greased springform pan (26 cm Ø) dusted with flour, press them down and refrigerate for 30 minutes. In the meantime, clean, wash and cut the rhubarb into large pieces. Mix the groats powder with 6 tablespoons of cold water and 75 g sugar.
Bring 340 ml water to the boil. Stir in the mixed porridge powder and bring to the boil again. Add rhubarb and fold in. Leave to stand for 5 minutes. Remove 2-3 pieces of rhubarb for decoration and put aside. Mix quark, buttermilk, 250 g sugar, lemon juice, eggs, pudding powder and semolina. Put half of the quark mixture on the crumb base and smooth it down. Spread the rhubarb compote evenly on top, leaving a 1 cm wide rim all around. Add the rest of the cheese mixture and smooth it down. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 50-60 minutes. Cover if necessary after 40 minutes of baking time. Remove the cake from the oven.
Spread the rhubarb compote evenly on top, leaving a 1 cm wide rim all around. Add the rest of the cheese mixture and smooth it down. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 50-60 minutes. Cover if necessary after 40 minutes of baking time. Remove the cake from the oven. Remove from the edge with a knife and let it cool down for about 2 1/2 hours. Cut rhubarb pieces into fine strips and decorate the cake with them. Dust with icing sugar