Melt the butter. Put the biscuits in a freezer bag and crumble them with a cake roll. Mix butter and crumbs. Line the bottom of a springform pan (26 cm Ø) with baking paper. Add crumb mixture and press down firmly. Chill for about 30 minutes.
In the meantime soak gelatine in cold water. Sort and wash the blueberries and put a handful aside. Put half of the blueberries and sugar into a tall mixing bowl and puree. Wash the lemon hot and rub dry. Grate peel and squeeze juice. Mix quark, lemon juice and rind and blueberry puree. Squeeze the gelatine and dissolve at low heat. Stir in some quark mixture, stir into the remaining quark and chill for about 10 minutes. Whip the cream until stiff. As soon as the quark mass begins to gel, fold in the rest of the blueberries and cream. Pour onto the cake base and spread until wavy.
Mix quark, lemon juice and rind and blueberry puree. Squeeze the gelatine and dissolve at low heat. Stir in some quark mixture, stir into the remaining quark and chill for about 10 minutes. Whip the cream until stiff. As soon as the quark mass begins to gel, fold in the rest of the blueberries and cream. Pour onto the cake base and spread until wavy. Chill for at least 6 hours. Decorate cake with blueberries and lemon balm