Preheat the oven to 350℉. Line a 9-inch cheesecake or springform pan with a parchment paper circle.
In a mixing bowl, stir together the Oreo cookie crumbs, granulated sugar, and melted salted butter. Pour the crumbs into the prepared pan. Use a spoon or a flat-bottomed measuring cup to firmly press the crumbs along the bottom and slightly up the sides of the pan. Bake the crust for 10 minutes until set and remove the pan from the oven to cool.
Turn the oven temperature down to 300℉.
In the bowl of a stand mixer fitted with a paddle attachment or in a large bowl with an electric hand mixer, cream the cream cheese and granulated sugar until smooth. Add in the dark cocoa powder and vanilla extract and mix on low speed until well combined. Add in the sour cream, melted semisweet chocolate, and heavy cream and continue to mix on low speed until smooth. Add one egg at a time, mixing in between each addition, until the cheesecake batter is combined and smooth.
Pour the cheesecake batter into the Oreo crust and spread evenly with an offset spatula. Bake the cheesecake in a water bath (see blog post for more information) for approximately 90 minutes until the edges are set and the center jiggles.
Turn off the oven and cool the cheesecake in the oven with the door closed for 30 minutes. After 30 minutes, crack the door of the oven and cool for another 30 minutes. Then, remove the cheesecake from the oven and cool to room temperature. Chill the cheesecake for at least 6 hours or overnight.