Preheat your oven to 325°F (163°C). Grease a 9-inch springform pan lightly and optionally line the bottom with parchment paper.

Crush Oreo cookies into fine crumbs and mix with melted unsalted butter until coated. Press firmly into the bottom of the springform pan and bake for 10-12 minutes.

Layer mini marshmallows evenly over the cooled crust.

Melt bittersweet or semisweet chocolate using a double boiler. In another bowl, mix softened cream cheese, granulated sugar, light brown sugar, cocoa powder, and cornstarch. Add the melted chocolate and mix until smooth.

Stir in sour cream, heavy cream, and vanilla extract. Add eggs one at a time on low speed.

Pour the chocolate filling over marshmallows in the crust and smooth the surface.

Place the springform pan into a larger baking dish and fill with boiling water halfway up the sides.

Bake at 300°F (149°C) for 1 hour and 20-30 minutes.

Turn off the oven and crack the door, letting the cheesecake cool inside for 1 hour.

Remove from the oven and cool completely on a wire rack.

Cover and refrigerate for at least 8 hours or overnight.

Heat marshmallow fluff with a splash of water until pourable and drizzle over the cheesecake.

Whip cream with powdered sugar; pipe around edges and sprinkle with mini marshmallows.