Stir the Oreo crumbs and melted butter until every crumb looks wet like wet sand. Press firmly into the bottom and halfway up the sides of a 9-inch springform pan. Use the bottom of a measuring cup to really pack it down. Freeze for 15 minutes while you make the filling.

Beat the room temperature cream cheese with the powdered sugar on medium speed until completely smooth and fluffy, about 3 minutes. Scrape the bowl twice. No lumps should remain.

Pour in the melted chocolate, cocoa powder, and vanilla. Beat on low until just combined, then increase to medium for 30 seconds until the mixture looks like thick chocolate frosting.

In a separate bowl, whip the cold heavy cream on medium-high speed until stiff peaks form, about 3-4 minutes. The cream should hold its shape when you lift the beaters.

Gently fold about one-third of the whipped cream into the chocolate mixture to loosen it, then fold in the remaining cream in two additions. Use a rubber spatula and cut through the center, turn the bowl, fold up. Stop when no white streaks remain.

Scrape the filling into the chilled crust and smooth the top with an offset spatula. Refrigerate uncovered for at least 5 hours, preferably overnight, until completely set and firm to the touch.

Before serving, whip the remaining 1/2 cup cream with 1 tablespoon powdered sugar until soft peaks form. Dollop or pipe on top of the cheesecake. Add chocolate shavings if desired. Run a thin knife around the edge before releasing the springform.