• Preheat the oven to 350F (180C or 160C on a fan-forced oven).
• Wrap the outside of a 9-inch springform pan with heavy duty aluminum foil. Wrap it at least 3 times so that the bottom and sides are covered and all the seems are covered.

Oreo Crust

• Crush the cookies (wafers and filling) in a food processor or blender until they're fine crumbs. Optionally, place the cookies in a freezer bag and crush with a rolling pin.
• Mix the crushed cookies with the melted butter.
• Press into the bottom and just slightly up the sides of the springform pan.
• Bake in the preheated oven for 8 minutes. Remove from the oven. Keep the oven turned on.

Cheesecake Filling

• As the crust is baking, make the filling.
• In a very large bowl, beat together the cream cheese and sugar until smooth and creamy. Then beat in the cocoa powder. Turn off the mixer and scrape down the sides and bottom of the bowl.
• With the mixer on a low speed, beat in the sour cream. Then mix in the buttermilk, vinegar and food coloring.
• Whisk the eggs together. Then carefully beat the eggs into the batter a little at a time - I did 3 additions.

Baking and Cooling

• Place the springform pan (still wrapped in foil) in the middle of the large roasting pan.
• Pour the cheesecake batter over the crust and smooth the top.
• Pour the boiling water into the roasting pan so that there's about 1/2 - 1 inch (about 1.5 - 2.5 cm) or water in the roasting pan surrounding the springform pan.
• Place the roasting pan (with the cheesecake inside) in the oven (it should still be at 350F from before). Bake for 55-65 minutes, or until the top looks just set and if you give the pan a gentle nudge the cheesecake still has a wobble like jello or pudding, but doesn't wobble like a liquid.
• Remove the roasting pan from the oven. Cool the cheesecake inside the roasting pan. When the water is room temperature, remove the cheesecake and place on a cooling rack to continue cooling.
• Once the pan feels room temperature, cover the top and place in the fridge to chill for at least 6 hours or overnight. This is necessary for the cheesecake to properly set.

Topping and Serving

• For the ganache, chop the chocolate into very fine pieces and place in a heatproof bowl.
• Heat 1/2 cup cream until almost boiling.
• Pour the hot cream over the chopped chocolate. Let it sit for 2-4 minutes, then whisk until smooth. If the chocolate hasn't quite melted, heat in the microwave on medium power for 30 seconds, then whisk.
• Remove the cheesecake from the fridge. Trace around the outside with a thin, sharp knife. Then unclamp the outer ring of the springform pan.
• If there's condensation on top of the cheesecake, blot with a paper towel to remove.
• Pour the ganache over the cheesecake, allowing it to drip over the edge. Place back in the fridge until ready to serve.
• Beat the remaining 1/2 cup cream until peaks form.
• Transfer the cream to a piping bag (I used a 1M tip). Pipe rosettes around the edges of the cheesecake.
• Crush or chop the Oreos, then sprinkle on top of the cheesecake.
• Slice with a thin, sharp knife. Be sure to slice through the crust.