Preheat oven to 325 F. Add cupcake liners to the muffin pan.
In a medium sized bowl, stir to combine the Oreo crumbs and melted butter. Divide the mixture between the cupcake liners. (I added about 1 tablespoon plus 1 teaspoon per muffin cup). Press mixture into the bottoms using a rounded tablespoon or bottom of a clean spice jar.
Bake the crusts 5 minutes then remove from the oven. Set them aside to cool while you make the filling.
Place the chopped chocolate in a microwave safe bowl. Microwave the chocolate for 20 seconds then stir. Continue microwaving and stirring until chocolate has melted completely. Set aside to cool slightly.
Reduce oven to 300 F.
In the bowl of an electric mixer fitted with the paddle attachment or in a large bowl using a hand mixer, mix the cream cheese and sugar on low until combined. Scrape down the sides of the bowl and mix in the cocoa until combined.
Beat in the vanilla extract, heavy cream and sour cream on low speed. Use a spatula to scrape down the sides and bottom of the bowl.
Mix in the egg followed by the egg yolk.
Use a spatula to stir in the melted chocolate until just incorporated.
Evenly distribute the batter in the prepared muffin tin, filling each cup until almost full.
Bake for 15 minutes then turn the oven off and leave the cheesecakes in the oven with the door closed for 5 minutes.
Remove the cheesecakes from the oven and place the pan on a wire rack to cool. Once they are cool to the touch, place the pan in the fridge to cool for at least 2 more hours.
If using ganache, spoon about 1/2 tablespoon of ganache on the top of each cheesecake. Use the back of a spoon or an offset spatula to spread it. Cover and place in the fridge for about 10 minutes to allow the chocolate ganache to set.