Preheat your oven to 350°F if you plan to toast the crust for added flavor (optional). Pulse the Oreo cookies into fine crumbs using a food processor, and mix them with melted unsalted butter until combined. Press this mixture into the bottoms of mini cheesecake pans. Set aside to firm up.

In a microwave-safe bowl, melt the semi-sweet chocolate chips in 30-second increments until smooth. In a separate bowl, blend cream cheese, granulated sugar, cocoa powder, and sour cream until creamy. Gradually fold in the melted chocolate to create the filling.

Whip the heavy whipping cream with powdered sugar until stiff peaks form. Gently fold the whipped cream into the chocolate mixture to maintain its airy texture.

Spoon the chocolate filling over the prepared crusts in the mini cheesecake pans, smoothing the tops. Cover with plastic wrap and refrigerate for at least 4 hours or ideally overnight.

Whip more heavy cream with cocoa powder and powdered sugar until stiff peaks form. Use this whipped cream to top each set cheesecake.

Release the cheesecakes from their pans and pipe or spread the whipped topping onto each. Optionally garnish with chocolate shavings or fresh berries. Chill briefly before serving.