Cook's Note: All the ingredients should be at room temperature before you start.
Preheat the oven to 325 degrees F. Process the crackers, peanuts, bittersweet chocolate chips, and butter in a food processor. Once it comes together in a clump, turn it into a springform pan and press into the bottom and up the sides to make the crunchy crust. Put it in the refrigerator while you make the filling.
Add all the filling ingredients to a food processor and pulse until it becomes a smooth mixture.
Pour and scrape the filling into the crust and bake for 1 hour, checking after 45 minutes. The top, only, should feel set and dry.
Take the cheesecakes out of the oven and set aside while you make the topping. Warm the sour cream and milk chocolate with the brown sugar in a small saucepan over a low heat. Gently whisk to blend in the chocolate as it melts, then take the pan off the heat.
Spoon and spread the topping very gently over the top of the cheesecake, being as careful as you so as not to break the surface. (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. Remove the cake from the oven and cool. Slice and serve.
Make the cheesecake up to 2 days ahead. Cool and chill as directed. Do not cover until completely cold, then cover with a plate or plastic wrap, making sure that the covering does not touch the surface. Unmold and serve as directed in the recipe. This will keep in the refrigerator for up to 4 days total.
The cheesecake can be frozen for 1 month. Make sure it is thoroughly chilled then wrap, still in its pan, in a double layer of plastic wrap and layer of aluminum foil. Thaw overnight in the refrigerator and eat within 2 days. A small amount of condensation may appear on surface of the cheesecake after thawing, but it is still fine to eat.
From Nigella Kitchen by Nigella Lawson. Copyright (c) 2010 Nigella Lawson. Published by Hyperion. All Rights Reserved.