PREP: Preheat the oven to 325 degrees F. Wrap a well-greased 9-inch springform pan in two layers of heavy-duty foil. Secure foil around the pan.
CRUST: In a small bowl, combine Oreo cookie crumbs and sugar and then stir in butter. Press firmly into the bottom of the greased and prepared springform pan -- don't go up the sides-- just press into the bottom of the pan. Place pan on a baking sheet. Bake for 10 minutes and then remove to a cooling rack. Meanwhile, melt the semi-sweet chocolate chips in the microwave in bursts of 15 seconds stirring in between each burst for 10 seconds until the chocolate is melted and smooth. Allow to cool to room temperature.
CHEESECAKE FILLING: In a very large bowl, beat the ROOM TEMPERATURE cream cheese and sugar until smooth at consistent low-medium speeds (beating in the same direction). Be careful to not overbeat or beat at high speeds -- this will cause the cheesecake to rise, then fall and crack. Use a spatula to scrape every bit of the melted and cooled chocolate and cocoa powder into the cheesecake filling mixture. Beat just to combine.
EGGS, VANILLA, AND SALT: In a small bowl, whisk together all of the eggs. Add them to the mixture along with the salt and vanilla extract. Beat on low speed until JUST combined. Use a spatula to scrape every bit of this mixture on top of the Oreo crust in the springform pan. Smooth the top of the cheesecake with the spatula. Place the springform pan in a large baking pan and add 1 inch of BOILING water to the pan surrounding the springform pan.
BAKE: Bake the cheesecake at 325 degrees for 55-70 minutes or until center is just set and top is not shiny. Cheesecake should ever so slightly jiggle in the very center (it will firm as it chills). If it jiggles a lot, it needs more time, check again in 5 minutes. Once fully baked, turn off the oven and fully open oven door. Pull the oven tray holding the cheesecake out and let the cheesecake stand there (still in water bath) to cool for 45 minutes. After 45 minutes, remove from water bath, carefully remove foil, and let stand at room temperature for 30 more minutes on a wire rack (we're slowly lowering the temperature of the cheesecake to avoid cracks). After 30 minutes, transfer to the fridge (covered) and chill for at least 8 hours (preferably overnight).
GANACHE: Place chocolate in a small bowl with the butter and heavy cream. Microwave in bursts of 20 seconds, stirring in between each burst for 10 seconds until the mixture is smooth. Let stand until it has cooled to room temperature before topping the cheesecake with the ganache. Spoon the ganache into the center of the cheesecake (keep it about an inch away from the edge) and smooth the ganache with the back of the spoon. Cover ganache-topped cheesecake and place in the fridge for an hour (or until the ganache is firmly set).
OPTIONAL WHIPPED CREAM: When ready to serve, combine the heavy whipping cream, powdered sugar, and vanilla extract in a stand mixer. Beat on high until soft peaks form and then spoon or pipe the whipped cream over the cheesecake slices.
SERVE: To get the perfect slices, you'll want to use a large, very sharp knife. Run the knife under very hot water, dry it off with a kitchen towel, and then make one quick and decisive cut. Repeat this process for each cut you make.
STORAGE: Chocolate Cheesecake can stand at room temperature for up to 2 hours, but after that, it needs to be refrigerated. To store: Place leftover cheesecake slices in an airtight container or individually wrapped in the fridge for up to 5 days (the crust softens more every day the cheesecake is stored). To freeze: Wrap individual cheesecake slices tightly with plastic wrap and then place all the individually wrapped slices in an airtight container. Freeze for 2-3 months. Thaw in the fridge overnight and leave out at room temperature for about 20-30 minutes before serving.