01 - Preheat the oven to 325°F (160°C). Grease a 9-inch springform pan and line the bottom with parchment paper. Wrap the outside of the pan tightly with aluminum foil if using a water bath.
02 - Combine the crushed chocolate sandwich cookies with the melted butter in a medium bowl until the mixture resembles wet sand. Press firmly and evenly into the bottom of the prepared springform pan. Bake for 10 minutes, then set aside to cool.
03 - In a large mixing bowl, beat the softened cream cheese and granulated sugar using a hand or stand mixer on medium speed until completely smooth and free of lumps. Add the peanut butter and blend until fully incorporated.
04 - Add the eggs one at a time, beating on low speed after each addition just until combined — do not overmix. Stir in the vanilla extract and sour cream until evenly distributed.
05 - Divide the filling mixture evenly into two separate bowls. Into one half, fold the melted dark chocolate until uniform in color. Leave the other half as the peanut butter mixture.
06 - Pour the chocolate filling over the cooled crust and spread into an even layer. Gently spoon the peanut butter filling on top. Using a butter knife or offset spatula, drag through both layers in figure-eight motions to create a marbled effect.
07 - Bake on the center rack for 50 to 60 minutes, until the edges are set and the center still has a slight jiggle when gently shaken. Avoid opening the oven door during baking.
08 - Turn off the oven and crack the door open slightly. Allow the cheesecake to rest inside the cooling oven for 1 hour. This gradual cooling helps prevent cracking.
09 - Transfer the cheesecake to a wire rack and cool to room temperature. Then refrigerate for at least 4 hours, or ideally overnight, before adding the topping.
10 - In a small saucepan, heat the heavy cream over medium heat until it just begins to steam — do not boil. Remove from heat, add the chopped dark chocolate, and let stand for 2 minutes. Stir until glossy and smooth, then blend in the peanut butter. Cool the mixture slightly until it reaches a spreadable consistency.
11 - Spread the ganache topping evenly over the chilled cheesecake. Garnish with chopped roasted peanuts if desired. Slice with a hot, clean knife for neat portions and serve.