Mix quark, 3 tablespoons lemon juice, 80 g sugar and 1 pinch of salt. Whip cream until stiff and fold in. Sort blueberries, wash and drain. Puree the berries, except for 1 tbsp for decoration, 3 tbsp lemon juice and 3 tbsp sugar with a hand blender.
Line a box form (approx. 1 1/2 l capacity; 25 cm long) with foil. Spread 4 tablespoons of quark cream into the mould, freeze for at least 20 minutes.
Spread 2 tablespoons of berry mixture on the frozen quark cream, freeze for at least 15 minutes. Repeat 4-5 times until the quark and berry mixture is used up.
Meanwhile melt butter. Crumble the pastry finely and stir into the butter. Spread over the last layer before freezing and press down lightly. Cover with foil and freeze for at least 6 hours, preferably overnight.
Let the cake thaw in the fridge for 45-60 minutes before serving. Then turn out of the mould. Cut the ice cream into slices and decorate with remaining berries.