Finely crumble the biscuits in the universal chopper. Melt the butter and stir in the cookie crumbs. Pour into the lightly oiled square springform pan (24 x 24 cm) and press down lightly as a base. Place the mould in a cool place for about 30 minutes.
Soak gelatine in cold water. Whip cream until stiff and chill. Mix cream cheese, quark, 200 g sugar, elderflower syrup and lemon juice. Squeeze the gelatine and dissolve at low heat.
Stir in 2 tablespoons of cream, then stir the gelatine mixture into the remaining cream. Fold in the cream.
Pour cream into the mould, smooth it down and chill the cake for at least 4 hours (preferably overnight).
Wash and clean the strawberries and cut them into smaller pieces according to size. Sprinkle with 4 tablespoons of sugar and chill for about 1 hour. Finely puree 1/4 of the strawberries with a hand blender. Remove the cake from the tin, spread strawberries and fruit puree on the cake.