Crush biscuits and mix with the butter. Line the base of a loose bottomed 20 cm flan case.Melt the chocolate, add to the marscapone cheese. Sweeten to taste. Put 150g blueberries in a pan with the amaretto liqueur and warm through for a few minutes. Blend and pass through a fine sieve. Return to the pan. Mix the arrowroot with a tsp of water and add to the pan. Warm and stir until thickened. Cool before mixing in the remaining berries. Top the cheesecake and set in the fridge.