Set the oven to 350°. Melt 1 cup chopped chocolate in the microwave or double boiler and set aside to cool.
Crust
Grease an 8" springform pan with butter. In a food processor, pulse Oreos until fine crumbs. Mix in the melted butter and press into the bottom of the springform pan. Bake for 8 minutes then set aside to cool.
Cheesecake
In a stand up mixer with the paddle attachment, beat cream cheese until smooth. Add sugar and beat well, scraping down the bowl as needed. Beat in sour cream, cornstarch, cocoa, salt and vanilla, again scraping down the bowl as needed. Add one egg at a time, mixing after each egg. Turn mixer down to low and slowly pour in the melted chocolate until just combined.
Prepping to bake
Time to build a water bath! Wrap the springform pan in heavy duty tin foil so it covers the bottom and comes up the sides of the pan. Place the tin foil wrapped springform pan inside a large roasting pan. Pour the cheesecake batter over the Oreo crust and smooth out the top. Add boiling water to the pan until it reaches about 1/2 an inch up the side of the springform pan. Check out my Cheesecake Tips for more details!
Baking
Bake at 350° for 45-55 minutes. The edges will be puffy but the center will still be jiggly. Once baked, open the oven door just a bit (you can place a wooden spoon in the way so it can't close) allowing the cake to rest inside for one hour, then pull the cake out and let it sit on the counter until it comes to room temperature before refrigerating.
Ganache
Once the cake is fully cooled, preferably overnight, the ganache can be made. Melt the chocolate, white sugar and heavy cream in the microwave or double boiler and pour over cheesecake layer in the springform pan. Allow the ganache to set in the refrigerator for one hour.
Caramel Sauce
For the caramel sauce, combine the sugar and 2 tbsp water in a small saucepan over medium heat. Stir to combine and then cook without stirring until sugar has turned a dark amber colour. (10+ minutes) Slowly add the warmed cream while whisking - the mixture will boil up all crazy like so be cautious. Simmer for 2 additional minutes. Remove from heat and whisk in vanilla and salt. Cool slightly.
Remove sides of springform pan before pouring caramel over cake. Garnish with salted peanuts. Try not to eat the whole thing in one evening! Yum!