1. Wrap the outside of a 9 inch springform pan with two layers of heavy duty foil, lightly grease the inside, and line the bottom with parchment paper. Preheat the oven to 325°F 160°C and place a large roasting pan on the middle rack.

2. Stir together the crushed chocolate sandwich cookies, granulated sugar, and melted butter until the crumbs are evenly moistened. Press the mixture firmly into the bottom of the prepared pan and bake for 8 to 10 minutes. Let the crust cool slightly while you prepare the filling.

3. In a large bowl beat the softened cream cheese until completely smooth with no lumps. Add the granulated sugar and beat until fluffy, then mix in the sour cream, heavy cream, and vanilla extract, scraping the bowl as needed.

4. Add the eggs and egg yolk one at a time on low speed, mixing just until each is incorporated. Sprinkle in the flour and mix on low only until the batter is smooth and combined, being careful not to over mix.

5. Transfer about 1 cup of the cheesecake batter to a small bowl and stir in the melted bittersweet chocolate chips until smooth. Pour this chocolate batter over the baked crust and spread it into an even layer.

6. Gently spoon the remaining vanilla cheesecake batter over the chocolate layer, smoothing the top with an offset spatula so the layers stay distinct. Tap the pan lightly on the counter to release any large air bubbles.

7. Place the springform pan in the roasting pan and carefully pour hot water into the roasting pan until it comes halfway up the sides of the springform pan, creating a water bath.

8. Bake the cheesecake for 60 to 70 minutes, until the edges are set and the center still has a slight wobble when the pan is gently shaken. Turn off the oven, crack the door, and let the cheesecake rest inside for 45 to 60 minutes.

9. Remove the cheesecake from the water bath, discard the foil, and run a thin knife around the edge to loosen it from the pan. Cool completely at room temperature, then cover and refrigerate for at least 6 hours or overnight.

10. For the ganache, heat the heavy cream in a small saucepan just until it begins to simmer. Pour it over the chopped chocolate and butter in a heatproof bowl, let sit for 1 minute, then whisk until smooth and glossy. Cool until slightly thickened but still pourable.

11. Release the chilled cheesecake from the springform pan and transfer it to a serving platter. Pour the ganache over the top, nudging it toward the edges so it drips gracefully down the sides.

12. To decorate, whip the chilled heavy cream with powdered sugar and vanilla extract until medium peaks form. Pipe or dollop the whipped cream around the outer edge of the cheesecake and top with chocolate candies or chopped candy bars if using.

13. Slice the cheesecake with a hot clean knife, wiping the blade between cuts, and serve chilled.