Melt the butter, remove from the heat and let it cool down. Coat the bottom of a springform pan (22 cm Ø) with oil. Wash the orange, rub dry and grate the peel thinly. Put 1 wafer roll aside for decoration. Finely mix the remaining rolls in the universal chopper. Mix butter, orange zest and biscuit crumbs and place on the bottom of the springform pan. Press the mixture and put it in a cool place
Soak gelatine in cold water. Halve the lemon and squeeze 1/2 lemon. Whip cream until stiff. Mix quark, yoghurt, mascarpone, lemon juice and 75 g sugar. Squeeze the gelatine and melt in a small pot. Stir in 3 tablespoons of cream, then stir into the remaining cream. Carefully fold in the cream. Pour the cream into the mould, smooth it down and chill for at least 4 hours
Peel the grapefruit so that the white skin is completely removed. Cut half the grapefruit into thin slices. Boil 100 g sugar and 100 ml water. Add the grapefruit slices and simmer for 2-3 minutes. Remove the grapefruit slices, drain and let them cool down. Remove the cake from the tin and decorate with grapefruit slices and the remaining pastry rolls
waiting time approx. 4 1/4 hours