1. Crush the OREO cookies into fine crumbs using a food processor or by placing them in a sealed bag and pounding with a rolling pin. Combine the crumbs with melted butter until the mixture has the consistency of wet sand.
2. Press the Oreo mixture firmly into the bottom of a greased springform pan to form the crust. Bake the crust at 350°F (175°C) for about 8 minutes to set it, then let it cool completely.
3. Beat the cream cheese in a large bowl using a stand mixer or handheld mixer until it becomes smooth and creamy.
4. Add sugar to the cream cheese and continue mixing until fully incorporated.
5. Add the eggs one at a time to the cream cheese mixture, beating at low speed to avoid overmixing.
6. Melt the semi-sweet or dark chocolate and allow it to cool slightly before mixing it into the cheesecake batter.
7. Add unsweetened cocoa powder, vanilla extract, and espresso powder (if desired) to enhance the chocolate flavor. Stir in sour cream or yogurt to create a rich, creamy filling.
8. Pour the chocolate cheesecake filling over the cooled crust, smooth the top with a spatula, and place the springform pan in a preheated oven.
9. Bake the cheesecake at 350°F for approximately 50-60 minutes. Ensure the center is set but still slightly jiggly. Optionally, bake using a water bath to prevent cracks.
10. Turn off the oven and slightly open the door to allow the cheesecake to cool gradually for an hour.
11. Transfer the cheesecake to a wire rack to cool completely, then refrigerate it for at least 4-6 hours, or ideally overnight, to set properly.
12. To prepare the chocolate ganache topping, heat the heavy cream until it is steaming (but not boiling), then pour it over chopped chocolate in a bowl. Let it sit for a moment before stirring until smooth.
13. Pour the ganache evenly over the chilled cheesecake, spreading it gently to cover the top completely. Refrigerate the cheesecake for an additional 30 minutes to set the ganache.
14. Before serving, carefully release the cheesecake from the springform pan and transfer it to a serving plate. Slice and enjoy with your favorite beverages or toppings!