- 100 g (31⁄2 oz) butter, plus extra for greasing
- 350 g (12oz) dark chocolate, in drops or broken into pieces
- 450 g (1lb) cream cheese
- 100 ml (31⁄2 fl oz) sour cream
- 3 eggs
- 175 g (6oz) caster sugar
- 150 g (5oz) hazelnuts, toasted, skinned and roughly chopped
- for the biscuit base
- 375 g (13oz) dark chocolate digestive biscuits
- 3 tblsp double or regular cream
- 75 g (3oz) butter, melted
- Preheat the oven to 170°C (325°F/Gas 3), and grease the sides of the cake tin (spring-form, 25cm or 10inch) with butter. Make sure the base of the cake tin is upside down, so there’s no lip and the cake can slide off easily when cooked.
- First make the biscuit base. Place the biscuits in a food processor and whiz until they form coarse crumbs. Alternatively, place the biscuits in a plastic bag and crush them using a rolling pin until finely broken down with only a few coarse pieces remaining.
- Tip the biscuit crumbs into a large bowl and mix with the cream and melted butter, then place in the cake tin, pushing the mixture down into the base to cover it as evenly as possible. Chill in the fridge while you make the topping.
- Place the butter and chocolate in a large heatproof bowl and set over a saucepan of just simmering water, stirring occasionally until melted and combined. When melted, remove from the heat and whisk in the cream cheese until smooth, then whisk in the sour cream.
- In a separate bowl using a hand-held electric beater or in an electric food mixer, whisk together the eggs and sugar for 2–3 minutes or until mousse-like in consistency, then turn off the machine and fold in the chocolate mixture along with the chopped hazelnuts.
- Add the mixture to the chilled biscuit base, spreading it in an even layer, then bake for 1 hour 30 minutes or until almost set. It should wobble a little if you gently shake the tin, while the top will be slightly cracked.
- Allow to cool in the tin on a rack for an hour, then chill, still in the tin, in the fridge for at least an hour before serving. Loosen around the edges using a small, sharp knife and unclip and remove the sides of the tin. Then use a long, sharp knife to loosen the bottom of the cake from the base and a palette knife or metal fish slice to slide the cake onto a plate.