For the Crust:

1. Preheat the oven to 350°F. Position a rack in the center of the oven.
2. Place the Oreos in a food processor fitted with the blade attachment and process into fine crumbs. Add the butter and salt and pulse until combined. The mixture will be crumbly, but should easily clump when pressed together.
3. Transfer the crumbs into an ungreased 8- or 9-inch springform pan. Press the crumbs firmly and evenly into the bottom and about halfway up the sides of the pan.
4. Transfer the pan to the oven and bake the crust for 8-10 minutes, until dry and fragrant. Remove the pan from the oven and let cool as you make the filling.

For the Cheesecake:

1. As the crust bakes, melt the chocolate in a double boiler on the stovetop or in a microwave-safe bowl in the microwave. Set aside to cool. Boil water in a kettle, and keep it hot as you prepare the filling.
2. Place the cream cheese and sour cream in the bowl of a stand mixer fitted with the paddle attachment. Mix together at medium speed until completely combined and smooth, about 1 minute.
3. Mix in the sugar, cocoa powder, vanilla extract, and salt at medium speed until completely combined, 1 minute more. Scrape the sides and bottom of the bowl and briefly remix.
4. With the mixer on low speed, add the eggs to the mixture one at a time, incorporating each one before adding the next. Scrape the sides and bottom of the bowl and briefly remix.
5. With the mixer still on low speed, add the melted chocolate and beat until combined, about 1 minute. Scrape the sides and bottom of the bowl and briefly remix.
6. Pour the batter onto the crust and spread it into an even layer.
7. Tightly cover the bottom and sides of the pan with 2 layers of aluminum foil. Place the pan inside a large baking or roasting pan with tall sides. Transfer to the oven and carefully fill the roasting pan halfway with the hot water.
8. Bake until the cheesecake is fully set around the edges with just a slight jiggle in the center, 60-75 minutes.
9. Remove the roasting pan from the oven and remove the springform pan from the water bath. Dispose of the aluminum foil.
10. Place the springform pan on a cooling rack and let the cheesecake cool at room temperature for 1 hour. Refrigerate the cheesecake for at least 6 hours to set, or preferably overnight.
11. When set, remove the ring from the springform pan, slice the cheesecake into wedges, and serve.