01 - Preheat oven to 325°F. 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 and slightly up the sides of the prepared pan. Bake for 10 minutes, then set aside to cool completely.
03 - In a large mixing bowl, beat the softened cream cheese and granulated sugar on medium speed until completely smooth and free of lumps, about 2 minutes. 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. Stir in the vanilla extract and sour cream. Avoid overmixing to prevent excess air in the batter.
05 - Divide the filling mixture evenly into two bowls. Fold the melted dark chocolate into one half, stirring until uniformly colored. Leave the other half as the plain peanut butter mixture.
06 - Pour the chocolate batter over the cooled crust and spread into an even layer. Spoon dollops of the peanut butter batter on top. Run a butter knife or offset spatula through the layers in gentle figure-eight motions to create a marbled effect.
07 - Place the pan on the center rack and bake for 50 to 60 minutes. The cheesecake is done when the edges are set and the center still has a slight jiggle when gently shaken.
08 - Turn off the oven, crack the door open a few inches, and let the cheesecake rest inside for 1 hour. This gradual cooling helps prevent cracks from forming on the surface.
09 - Transfer the cheesecake to the refrigerator and chill for at least 4 hours, or ideally overnight, until fully set and firm throughout.
10 - Heat the heavy cream in a small saucepan over medium heat until it begins to steam (do not boil). Remove from heat, add the chopped dark chocolate, and let sit for 1 minute. Stir until a glossy ganache forms, then blend in the peanut butter until smooth. Cool slightly until the mixture thickens to a spreadable consistency.
11 - Spread the topping evenly over the chilled cheesecake. Garnish with chopped roasted peanuts if desired. Slice with a sharp knife dipped in hot water for clean portions and serve immediately.