Crust

1. Preheat oven to 350F/180C. Line a 9-inch square pan with heavy-duty foil so that it overhangs on the sides, then grease the foil heavily and set it aside.
2. In a food processor, crush the Oreos till they become a fine crumb. Add melted butter, stirring until fully combined. Press into the bottom of the prepared pan, use a cup to tightly press the crumbs into place. Bake for 5 minutes then set aside while you prepare the filling.

Cheesecake

1. In a Bain Marie or in the microwave melt the chocolate just until the majority of chocolate is melted- whisk together until everything is melted- set aside to cool. Be careful not to overheat the chocolate once chocolate burns there is no going back. 
2. In the bowl of an electric mixer fitted with a paddle attachment or large bowl with a handheld mixer beat cream cheese for 2 minutes on medium to high speed. Add granulated sugar and sift cocoa powder in then beat for another minute.
3. Add in the chocolate, sour cream, heavy cream, and vanilla extract mixing until fully combined- make sure to scrape the bottom of the bowl so you have no white marks. Add in eggs and yolk one at a time, and mix until just combined. You should have a creamy batter with no lumps. Pour on top of the crust and use a spatula to spread into an even layer.
4. Take a larger pan, a 9X13 pan, or a roasting pan and fill it halfway with warm/hot water place it on the bottom rack in the oven and place the cheesecake pan on the top rack above the water bath rack.
5. Bake for 28 to 35 minutes, the middle of the cheesecake should jiggle just a little bit. You can use an internal thermometer to check the internal temperature is 145F/63C. Turn the oven off and crack the door open and let cool in the oven for 30 minutes then transfer to a wire rack on the counter for one hour then wrap with plastic wrap and chill for a minimum of 4 hours or overnight. 

Topping

1. Slice the cheesecake into squares, I did 5X5 rows for 25 pieces but for larger slices, I do 3X3. Top with fresh whipped cream, I used a Wilton 1M tip then use a Microplane zester to grate chocolate shavings on top. Store the cheesecake in the fridge for up to 5 days.