Butter and line the base of a 20cm springform cake tin. Blitz the biscuits in a food processor into fine crumbs, then add the butter and blitz again. Tip the mixture into the tin and press down firmly with the back of a spoon. Chill while you make the filling.
Gently mix the mascarpone, soft cheese, icing sugar and cocoa until smooth. In a separate bowl, whip the cream until it is stiff, then fold into the mascarpone mixture. Spoon the mixture over the biscuit base and level the top. Chill for 1 hr.
Warm the caramel in a pan a little so it is easily spreadable, if you overdo it, wait until it cools again. Pour it onto the cheesecake, spread gently and chill. Meanwhile, melt the chocolate carefully in a bowl set over a pan of simmering water or in a microwave. Drizzle it back and forth over the caramel, then add the sprinkles or stars in whatever pattern you like. Chill until ready to serve, then carefully remove the tin and lining and slide the cheesecake onto a serving plate.