• Preheat the oven to 325F. Line a 9x5 inch (23x13 cm) loaf pan horizontally with parchment paper so that there's an overhang around the sides.

Oreo Crust

• Crush the cookies (wafers and filling) until fine crumbs. This can be done in short bursts in a food processor or by placing the cookies in a freezer bag and crushing with a rolling pin.
• Stir together the cookie crumbs and butter.
• Add the prepared loaf pan and press down into an even layer.
• Bake in the preheated oven for 8 minutes. Remove from the oven, leaving the oven turned on.

Chocolate Cheesecake Filling

• Finely chop the chocolate and place in a heatproof bowl.
• Heat the cream until almost boiling - either in 30 second intervals on medium power in the microwave or using a double boiler. Pour the hot cream over the chopped chocolate. Let sit for 2-4 minutes, then whisk until smooth. If the chocolate hasn't fully melted, return to the microwave for 30-second intervals on medium power, whisking between each interval. Alternatively, return to the double boiler. Set aside
• In a medium bowl, beat together the cream cheese, sifted cocoa and sugar. Beat until smooth and creamy. You should no longer see lumps of cream cheese. Be sure to turn off the mixer and scrape down the bottom and sides of the bowl to remove lumps.
• Bea the melted chocolate and cream mixture into the cream cheese mixture until evenly combined.
• Whisk in the egg. This can be done by hand with a wire whisk or with the mixer on a low speed. Stop mixing when you no longer see pieces of egg.
• Pour the cheesecake batter into the loaf pan and smooth the top.

Baking and Cooling

• Place the loaf pan in the middle of a larger pan (I used a 9x9 inch pan).
• Pour about 1 inch (2-3 cm) of boiling water into the larger pan so that the loaf pan containing your cheesecake is surrounded by water.
• Place in the middle of the preheated oven and bake for about 35-40 minute or until the top looks almost set and the cheesecake has a slight wobble still if you gently nudge the pan.
• Cool the cheesecake (still in the pan) until it's room temperature. Remove the loaf pan from the larger pan, cover with foil and place in the fridge to chill. Chill the cheesecake for at least 4 hours, or overnight.

Chocolate Ganache Topping & Serving

• Finely chop the chocolate and place in a heatproof bowl.
• Heat the cream until almost boiling (as you did for the cheesecake batter). Pour the hot cream over the chopped chocolate. Let sit for 2-4 minutes then whisk until smooth.
• Blot any condensation off the top of the cheesecake and pour the ganache over the cheesecake. Place back in the fridge until ready to serve.
• When ready to serve, lift the cheesecake out of the pan using the overhang of the parchment paper. Place on a cutting board. Slice with a sharp knife, being sure to slice through the crust.