In a medium bowl, combine crushed Oreo cookies and melted unsalted butter until the mixture resembles wet sand. Press this mixture firmly into the bottom of a 13 x 9-inch casserole dish. Chill the crust in the refrigerator for 15-20 minutes.

In a large mixing bowl, beat the softened cream cheese and powdered sugar until smooth. In another bowl, whip the heavy cream with hot chocolate mix until stiff peaks form. Gently fold the whipped cream into the cream cheese mixture until combined. Spread this mousse evenly over the chilled crust and return to the refrigerator for 30 minutes.

In a mixing bowl, whisk together instant chocolate pudding mix and milk for about 2 minutes until thickened. Fold in mini marshmallows and spread this layer over the cheesecake mousse. Chill the dessert for about 1 hour.

Once the pudding layer is slightly firm, spread whipped cream over it. Sprinkle additional mini marshmallows and drizzle chocolate syrup. Chill for at least 30 minutes before serving.