Make the Crust:
Place whole Oreo cookies with filling in a food processor and grind into fine crumbs.
Stir in melted butter until combined.
Press the mixture into the bottom of a 13 x 9-inch casserole dish.
Place in the fridge or freezer to firm up while preparing the next layer.
Hot Chocolate Cheesecake Mousse Layer:
In a large bowl, beat softened butter and cream cheese with vanilla and powdered sugar until creamy and smooth.
In a separate bowl, beat heavy whipping cream and instant hot cocoa mix until stiff peaks form.
Fold half of the hot chocolate whipped cream into the cheesecake mixture, then fold in the remaining half, scraping the bowl well to ensure everything is incorporated.
Spread the cheesecake mousse over the Oreo crust and set it in the fridge or freezer to firm.
Chocolate Pudding Layer:
In a medium bowl, whisk together the instant chocolate pudding mix and milk until it begins to thicken.
Stir in the mini marshmallows, then spread the pudding layer over the hot chocolate mousse layer.
Place the dish back in the fridge or freezer to set.
Topping:
Once the pudding layer has set, beat the heavy whipping cream and powdered sugar until stiff peaks form.
Spread the whipped cream over the pudding layer.
Place in the fridge for a few hours, or preferably overnight, to set.
Serve:
Before serving, scatter 2 cups of mini marshmallows over the whipped cream layer.
Drizzle with melted chocolate, Nutella, or your favorite chocolate topping.
Store leftovers in the fridge.