Melt the butter in a small pot and let it cool down a little. Crumble the biscuits finely and mix with the melted butter. Place the springform pan rim or cake ring (26 cm Ø) on a cake plate. Add the crumb mixture and press it down to a flat base. Chill for about 30 minutes. Soak gelatine in cold water.
Stir the quark, lemon juice and zest, sugar and vanillin sugar until smooth. Whip cream until stiff. Squeeze the gelatine, dissolve and mix with 2 tablespoons of quark mixture. Stir into the remaining quark, fold in the cream. Spread the cream on the base and smooth it down. Chill the cake for about 6 hours, preferably overnight. Finely puree the mandarin oranges with the juice. Put the cake glaze in a pot. Gradually add the mandarin puree while stirring, bring to the boil briefly. Remove cake from the ring. Paint a pattern on the cake with the glaze. Spread a thin layer of the icing on the edge.
Finely puree the mandarin oranges with the juice. Put the cake glaze in a pot. Gradually add the mandarin puree while stirring, bring to the boil briefly. Remove cake from the ring. Paint a pattern on the cake with the glaze. Spread a thin layer of the icing on the edge. Sprinkle the cake with pistachios and let it set. Serve decorated with mandarins