Preheat the oven to 170C/325F/Gas 3.
For the base, process the biscuits, peanuts, dark chocolate chips and butter in a food processor. Once the mixture comes together in a clump, turn it out into a 23cm/9in springform tin and press it into the bottom of the tin and up the sides, using your fingers, to make a crunchy crust. Chill in the fridge while you make the filling.
For the filling, process the cream cheese, eggs and egg yolks, sugar, soured cream and peanut butter in a food processor, whizzing to a smooth mixture.
Pour and scrape the filling mixture into the cheesecake base in the chilled springform tin and cook for 1 hour, checking after 50 minutes. The top – only – should feel set and dry. When cooked, take the cheesecake out of the oven while you make the topping.
For the topping, warm the soured cream and chocolate chips with the brown sugar gently in a small saucepan over a low heat, whisking to blend in the chocolate as it melts. Once the mixture is melted and well combined, 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 completely in its tin, then cover and put into the fridge overnight. When you are ready to eat the cheesecake, take it out of the fridge slightly in advance, 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.
All ingredients should be at room temperature before you start. The cheesecake can be made up to 2 days ahead. Once made, cool and chill as directed. Do not cover until completely cold, then cover with a plate or cling film, making sure that the covering does not touch the surface of the cheesecake. Chill in the fridge until needed, then unmould and serve as directed. The cheesecake will keep in the fridge for up to 4 days in total.
The cheesecake can also be frozen for up to 1 month. To freeze, chill thoroughly, then wrap, still in its tin, in a double layer of cling film and one layer of aluminium foil. Defrost overnight in the fridge and eat within 2 days. Some condensation may appear on the surface of the cheesecake when defrosting but it's fine to eat.