Line a 9- by 12-inch baking dish with foil, allowing the foil to hang over the sides. Spray the foil with cooking spray.

Combine the graham cracker crumbs with the melted butter and press firmly into the bottom of the prepared baking dish. Refrigerate while making the filling.

Using an electric mixer fitted with a paddle attachment, whip the cream cheese until smooth. Add the marshmallow fluff and beat until fully combined, scraping down the sides and bottom of the bowl a few times. Spread the mixture evenly on top of the crust. Sprinkle the chocolate chips evenly on top, then sprinkle with the mini marshmallows.

Position the top rack of the oven 4-6 inches away from the broiler. Preheat the broiler to high. Place the bars under the broiler just until the marshmallows plump up and start to brown, about 1 minute. Cover and refrigerate until firm, at least 4 hours or up to 2 days.

Using the foil, remove the cheesecake from the pan and cut into bars.