Crust

1. Grease a 6-inch springform pan set aside. Preheat oven to 325F/163C. In a food processor, crush the Oreos till they become a fine crumb. Add melted butter, stirring until fully combined. Press into bottom and slightly up upsides of prepared pan, use a cup to tightly press the crumbs into place. Place in the fridge while you make the cheesecake.

Cheesecake

1. In a Bain Marie or in the microwave melt the chocolate and heavy cream together, until chocolate is all melted- whisk together- set aside to cool.
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 beat for another minute.
3. Add in the chocolate mixture, cocoa powder, sour 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 into a prepared pan.
4. Place the springform pan in a larger cake pan- place the cake pan in a roasting pan and fill the roasting pan with hot water halfway up the sides of the cake pan. This prevents ANY water from getting in the cheesecake and you don't have to wrap everything with foil.
5. Bake for 55 to 65 minutes, the middle of the cheesecake should jiggle just a little bit. Turn oven off and let cool in the oven for 45 minutes to one hour then take out of roasting pan and let cool on the counter for one hour then wrap with plastic wrap and chill for a minimum of 6 hours or overnight. 

Topping

1. In a small pot over low heat warm the heavy cream, heat the cream until just before it boils. Pour over the chopped chocolate, in a heatproof bowl. Let sit for 5 minutes, then whisk together to create a ganache.
2. Pour over the chilled cheesecake and use an offset spatula to gently push down the sides for drips. Place back in the fridge for 30 minutes or 15 in the freezer.
3. Once the ganache is set add the whipped cream and if desired chocolate shavings. To create shavings use a vegetable peeler to peel chocolate from a large bar. Store the cheesecake in the fridge for up to 5 days.