Make the Crust: Preheat oven to 325℉. Line a muffin pan with 12 cupcake liners and set aside. Use a food processor or blender to finely crush 18 Oreos. Melt the butter (about 20-30 seconds in the microwave). Mix together the melted butter and crushed Oreos in a medium-sized mixing bowl.
Scoop 2 tablespoons of the crushed Oreo mixture into the bottom of each lined muffin tin. Press down with your fingers to form a solid crust. Bake the Oreo crust in the preheated oven for 5 minutes.
Make the Cheesecake: Melt the 4 ounces of semi-sweet chocolate by microwaving in 30-second increments, whisking in between. Once the chocolate is melted, set it aside. In a large mixing bowl, use an electric mixer to cream the cream cheese and Greek yogurt for about 2 minutes, or until smooth. Add the melted semi-sweet chocolate and granulated sugar.
Add the eggs one at a time. Then add the vanilla extract and cocoa powder. Use a citrus zester to zest the orange into the batter. Then juice the orange and add the juice into the batter. Mix all ingredients until the batter is smooth.
Bake the Cheesecake: Fill each lined muffin cup with cheesecake batter all the way to the top. Bake the mini cheesecakes for 20 minutes. After 20 minutes, turn the oven off and open the oven door. Keep the muffin pan in the oven as it cools down. Let the mini cheesecakes cool for 20-30 minutes before removing the pan and adding the chocolate ganache.
Make the Chocolate Ganache: Add the semi-sweet chocolate chips and heavy cream to a microwave-safe bowl. Microwave in 30-second increments, whisking each time until the chocolate and cream are melted together. Whisk until smooth. Top each mini cheesecake with a tablespoon of chocolate ganache. Transfer the muffin pan of cheesecakes to the fridge to chill for 3-5 hours before serving.
Make the Chocolate Whipped Cream (Optional): To make the chocolate whipped cream, add cold heavy cream, powdered sugar, cocoa powder, and vanilla extract to a cold, large mixing bowl. Use an electric mixer set on medium-high and mix the ingredients for about 5 minutes, or until the liquid turns to a light whipped cream texture.
Transfer the whipped cream to a piping bag equipped with a piping tip and pipe the chocolate whipped cream onto each cheesecake. If you have an extra orange, you can zest over the mini cheesecakes as added decoration. Serve immediately, or transfer back into the fridge until it's time to serve the mini cheesecakes.