Finely crumble the biscuits in the universal chopper. Melt butter and stir in the cookie crumbs. Place the rim of a square springform pan (24 x 24 cm; alternatively round, 26 cm Ø) on a cake plate lightly smeared with oil. Pour in the crumb mixture, press it down as a base and chill for about 30 minutes.
Soak gelatine in cold water. Whip the cream until stiff. Mix cream cheese, quark, 200 g sugar, elderflower syrup and lemon juice. Squeeze the gelatine and dissolve at low heat. First mix with 2 tablespoons of cream, then stir into the rest of the cream. Fold in the cream. Spread the cream cheese mixture on the crumb base, smooth it down and put it in a cool place 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. Puree 1⁄4 of the strawberries with a hand blender. Loosen the edge of the springform pan, spread the strawberries and fruit puree on the cake.