Preheat the oven to 325℉. Wrap a well-greased, 9-inch springform pan in two layers of heavy-duty aluminum foil. Secure foil around the pan.
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.
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.
In a very large bowl, beat the room temperature cream cheese and sugar in a circular motion and with consistent medium-low speed until smooth. Be careful not to overbeat or beat too fast or your cheesecake will fall and crack after baking. Use a spatula to scrape every bit of the melted and cooled chocolate into the cheesecake filling mixture. Add cocoa powder. Beat until just combined.
In a small bowl, whisk the eggs together. Add them to the filling mixture along with 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 baking pan.
Bake the cheesecake at 325℉ for 55–70 minutes, or until the center is set and the top is no longer shiny. The center should have a slight jiggle, as it will firm up as it cools. If it jiggles a lot, it needs more time—check again in 5 minutes. Once fully baked, turn off the oven and open the door completely. Carefully pull the oven tray with the cheesecake out and let it cool in the water bath for 45 minutes. Afterward, remove it from the water bath, carefully take off the foil, and let it sit at room temperature on a wire rack for 30 minutes. This gradual cooling helps prevent cracks. Finally, cover the cheesecake and transfer it to the fridge to chill for at least 8 hours, preferably overnight.
Place chocolate for the ganache 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 fully set.
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.
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.
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Notes
Cheesecake Tips and Troubleshooting:
Use room-temperature cream cheese. Room-temperature cream cheese becomes creamier when it is mixed, and it incorporates much better with the other ingredients in this recipe. If your cream cheese is too cold, it tends to stay lumpy, and the batter doesn’t fully mix. You’ll also likely be tempted to overbeat the batter to get rid of the lumps which is not good for the cheesecake.
Use room-temperature eggs. The more even the temperature of the ingredients, the better they will incorporate and the less chance you have of getting cracks in the cheesecake.
Don't overmix the batter. It’s important to mix the cream cheese enough to remove the lumps and to combine the ingredients sufficiently but overmixing the batter will introduce too much air into the mixture which causes the cheesecake to rise too quickly in the oven and then fall and create cracks in the cheesecake.
Mix the cheesecake batter at low and consistent speeds. This reduces the risk of adding in too much air which will cause cracks on top. I recommend beating the cheesecake batter in the same direction for the duration of mixing.
Allow the cheesecake to cool completely at room temperature. It might not seem like a big deal to speed up the chilling process of the cheesecake and put it in the fridge right out of the oven, but this will cause condensation to form on top and change the texture of your cheesecake.
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. 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.