Preheat the oven to 475 degrees F.
Place a large piece of foil over the bottom of the cheesecake pan. (it should cover about 2/3 of the outside sides of the pan as well) Line the inside bottom of the pan with parchment paper. To do this easily, flip the pan upside down and trace a circle on a piece of parchment with a pencil. Cut along the outside of the traced circle and place the finished circle in the bottom of the cheesecake pan. In a food processor(see note if you don't have one), pulse the Oreo cookies, until they are fine crumbs. Place the crumbs in a making bowl and pour in the melted butter. Using your hands or a fork mix until the crumbs are completely coated in butter. Oreo cookies tend to soak up a lot of butter, which leaves you with a wet and sticky crust, don't worry this is how it should be!
Press the crust mixture into the bottom and roughly 1 inch up the sides of the springform pan. If your Oreo crumbs are too sticky try buttering your hands before you start or wearing gloves.
Place the pan in the freezer, to help firm up the crust while you prepare the cheesecake filling.
In a large bowl combine the softened cream cheese, sour cream, sugar, and vanilla extract. Beat with an electric mixer at high speed, until the mixture is smooth and creamy, and there are no lumps of cream cheese. Scrap down the sides of the bowl and mix again. Add the cocoa powder a 1/4 cup at a time, be sure to beat on low speed to avoid getting cocoa powder everywhere! Taste the cheesecake mixture after each 1/4 of cocoa powder and add more until the cheesecake is to your liking. I usually add 1/2 of cocoa powder, but feel free to add more or less depending on how chocolatey you want it.
Add the eggs and beat the mixture on medium speed until completely combined. Scrap down the sides of the bowl and mix again
Remove the pan from the freezer, and using a rubber spatula scrap the chocolate cheesecake filling into the pan. Jiggle the pan around a little to smooth out the top of the cheesecake. Place the cheesecake in your water bath pan of choice(I use a large cast iron skillet) and fill it with 2-3 inches of water.
Place the pan in the hot oven and bake for 12 minutes.
After 12 minutes reduce the oven temperature to 350 degrees F, and continue baking for 40-50 minutes or until the middle is set and the top is slightly browned.
Remove the cheesecake from the oven(and the water bath) and allow it to cool at room temperature for 30 minutes to 1 hour.
Place the cheesecake in the fridge and allow it to chill for 3-4 hours or overnight.
Once the cheesecake has chilled, release the springform pan, and transfer the cheesecake to a plate(though you can also serve it from the bottom of the pan) top your finished cheesecake with chopped Oreo cookies, chocolate chips, and a drizzle of melted chocolate.
Cut into 12 slices and enjoy!