Preheat the oven to 170°C/gas mark 3, then process the biscuits, peanuts, dark chocolate chips and butter for the base in a food processor. Once it comes together in a clump, turn it out into a springform tin and press into the bottom and up the sides to make the crunchy crust. Put in the fridge while you make the filling.
Process the filling in the cleaned or wiped-out processor bowl, putting in the cream cheese, eggs and egg yolks, sugar, sour cream and peanut butter and whizzing to a smooth mixture.
Pour and scrape the filling into the base in the chilled springform tin and cook for 1 hour, though check after 50 minutes. The top - only - should feel set and dry.
Take the cheesecake out of the oven while you make the topping. Warm the sour cream and chocolate with the brown sugar gently in a small saucepan over a low heat, whisking to blend in the chocolate as it melts, and then take off the heat.
Spoon and spread the topping very gently over the top of the cheesecake, being as careful as you can in case you break the surface of the cheesecake. (Not that anything bad will happen; you'll just have chocolate marbling the cake a bit.) Put it back in the oven for a final 10 minutes.
Once out of the oven, let the cheesecake cool in its tin and then cover and put into the fridge overnight. When you are ready to eat the cheesecake, take it out of the fridge, just to take the chill off: this will make it easier to spring from the tin. Don't let it get too warm, though, as it will become a bit gooey and be hard to slice.
MAKE AHEAD NOTE:
Make the cheesecake up to 2 days ahead. Cool and chill as directed. Do not cover until completely cold, then cover with plate or clingfilm, making sure that covering does not touch surface. Unmould and serve as directed in recipe. This will keep in fridge for up to 4 days total.
The cheesecake can be frozen for 1 month. Chill thoroughly, then wrap, still in tin, in double layer of clingfilm and 1 layer of foil. Defrost overnight in fridge and eat within 2 days. Some condensation may appear on surface on defrosting but it is fine to eat.
Recipe from Nigella: Kitchen by Nigella Lawson (published by Chatto & Windus).