Blitz the digestive biscuits in a food processor (or smash them in a bag, using a rolling pin) until fine. Mix with the melted butter, then pat down evenly over the lined base of a 20cm (8in) Springform cake tin. Transfer to the fridge to set.
Whisk the mascarpone and Philadelphia together until smooth and set aside. Melt the dark chocolate in a heatproof bowl