All ingredients should be 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.
Mix all of the filling ingredients until smooth. 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 cheesecake 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 can, so as not to break the surface (not bad, but you will have marbling). Put it back in the oven for a final 10 minutes. Remove the cake from the oven and cool. Refrigerate at least 4 hours or overnight before serving.
NOTE: With cheesecakes I have found that even though a recipe says a certain cooking time, you may need longer, or less I suppose in some cases, so it's a bit of trial and error. If it's too wet or too dry this time, don't give up, try again. It's worth it in the end!