Melt the butter in a saucepan. Add the crushed biscuits and mix well. Divide the mixture between two tart tins, 10 cm (4 inches) across, and press down to form the cheesecake bases.
Place the cream cheese, caster sugar, mascarpone and chocolate in a small pan and warm over a gentle heat, stirring until the mixture is melted and blended.
Remove from the heat, allow to cool and then beat in the egg and the yolk.
Divide the chocolate mixture between the two tart tins, place them on a baking sheet and cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 45 minutes or until set. Remove from the oven and leave the cheesecakes to cool before transferring them to the refrigerator. Chill until required, then decorate with a dollop of crème fraîche and chocolate shavings.