Put the biscuits and chocolate in a food processor or blender and process to make fine crumbs. Mix with the melted butter and press the mixture gently over the base of a 20 cm (8 inch), round, springform cake tin. Place in the freezer to set while making the cream cheese mixture.
Beat together the cream cheese, mascarpone, sugar, mint liqueur or extract and food colouring in a large bowl. Stir in 40 g (1½ oz) of the chocolate chips and spoon the mixture over the biscuit base, smoothing with the back of a spoon.
Place in the refrigerator to chill for about 1 hour.
Loosen the edge with a knife, then remove the cheesecake from the tin carefully. Scatter over the remaining chocolate chips, roughly chopped.