Preheat the oven to 350℉. Line a 9x13-inch baking pan with foil (or parchment paper), leaving an overhang of foil to help you lift the cheesecake out of the pan later. Spray the foil with nonstick spray.
PREPARE THE CRUST:
Put the OREO cookies into the bowl of a food processor (no need to remove the creme filling). Process a few times, until the cookies have turned into crumbs. Pour in the melted butter, and process again until combined.
Scrape the crumb mixture into the prepared baking pan. Press the crumbs into an even layer. Bake for 10 minutes. Remove, and let cool while you make the filling.
MAKE THE FILLING:
Reduce the oven temperature to 325℉.
Add the chopped bittersweet chocolate to a heatproof bowl set over a pan of simmering water. Do not allow the bowl to touch the water. Melt the chocolate, then remove from the heat and set aside.
In a large bowl, use an electric mixer to beat the cream cheese until it's smooth and creamy. Beat in the sugar. Then beat in the cocoa powder and vanilla extract.
Add the eggs and heavy cream, and beat briefly- just until combined. Don't over-beat. Gently fold the melted chocolate into the cream cheese mixture with a rubber spatula until its incorporated into the mixture.
Pour the filling on top of the baked OREO crust. Smooth it into an even layer. Place the pan into the oven for 30 minutes. The edges of the cheesecake will look set, but the middle will still look slightly wobbly. After 30 minutes, turn the oven OFF, but leave the pan in the oven with the oven door open slightly. After an hour, remove the pan from the oven. Cover the pan with plastic wrap and refrigerate for at least 4 hours.
ADD THE GANACHE TOPPING:
Add the chopped chocolate to a bowl. Heat the cream in a small saucepan until simmering. Then pour the hot cream over the chocolate in the bowl. Let it sit for 5 minutes. Whisk the chocolate and cream together slowly until you have a thick and glossy ganache.
Pour the ganache over the set cheesecake and spread with a flat spatula to cover the top evenly with the ganache. Let it sit at room temperature for about an hour. Use the edges of the foil to lift the cheesecake out of the pan. Carefully peel away the foil from the cheesecake. Slice and serve.