• Preheat the oven to 170°C/150°C fan/325°F/as mark 3
• Grease & line and 22cm diameter spring form tin

For the base

• Using a food processor, grind the plain chocolate digestives into fine crumbs
• Add in the melted butter and combine well
• Tip the mixture into the prepared tin pressing down firmly & evenly using a cold metal spoon to form a level base
• Put the base into the oven for 10 minutes, when cool put it into the fridge to set

For the filling

• After washing out the processor bowl, use it to combine the mascarpone, eggs, yolk, sugar, peanut butter, soured cream and cornflour to a smooth consistency
• Pour the mixture into the over the base and bake for 30-40 minutes until just set
• Allow to cool for half an hour on a wire rack before putting the tin into the fridge to chill

For the topping

• In a small saucepan, gently warm the soured cream, chocolate and sugar stirring to form a smooth sauce
• Pour the mixture on top of the cold cheesecake and allow it to set in the fridge
• When cooled & set, run a sharp knife around the edge of the tin before easing the cake carefully
• Finish with a sprinkling of mixed chopped toasted nuts (optional)