Start by preheating your oven to 350°F. Prepare a 9x9 inch baking pan by lining it with parchment paper, leaving extra on the sides to lift the bars out later.

To make the crust, mix together the crushed Oreo cookies with the melted butter in a medium bowl until the crumbs are evenly coated. Press this mixture firmly into the bottom of the prepared pan. Bake for about 10 minutes, then remove from the oven and set aside. Lower the oven temperature to 325°F.

For the filling, place the chopped chocolate and 1/4 cup heavy cream in a microwave-safe bowl. Heat in the microwave in 30-second bursts, stirring after each one, until the chocolate is fully melted and the mixture is smooth. Let it cool slightly.

In a large mixing bowl, beat the softened cream cheese with an electric mixer on medium speed until smooth. Gradually add in the sugar and cocoa powder, continuing to beat until well combined.

Reduce the mixer speed to low and add the sour cream and vanilla extract. Mix until incorporated. Beat in the eggs and egg yolks one at a time, ensuring each is fully mixed in before adding the next. Finally, fold in the melted chocolate mixture until smooth.

Pour the prepared batter over the cooled crust and smooth it out. Bake for 40-45 minutes, until the center is set but still slightly jiggly. Turn off the oven and crack the door open slightly, letting the cheesecake cool in the oven for about an hour. Then transfer to a counter to cool to room temperature before covering and chilling in the refrigerator for at least 3 hours, or overnight.

To prepare the topping, combine the chopped chocolate and 1/2 cup heavy cream in a microwave-safe bowl. Heat in the microwave in 30-second intervals, stirring until the mixture is smooth and creamy. Pour this ganache over the chilled cheesecake, spreading it evenly. Return the cheesecake to the fridge for another 30-60 minutes until the topping is set.

When ready to serve, use the parchment overhang to lift the cheesecake from the pan. If desired, sprinkle with flaky sea salt before cutting into squares. Serve chilled and enjoy!