Preheat oven to 325 degrees. Lightly spray a 9" x 13" baking dish with cooking spray. (If you want to toast the marshmallows under the broiler later, use a metal pan.)
Dump dry cake mix, eggs and water into baking dish. Whisk until well blended, scraping down sides.
Stir in 1 cup mini marshmallow, spreading evenly.
Bake about 20 minutes until nearly done. Sprinkle graham cracker crumbs, chocolate chips and remaining 1 1/4 cups marshmallows over top and bake another 10 minutes or until toothpick inserted in center comes out clean. Place under broiler briefly to toast marshmallows.