Preheat oven to 350 degrees. Prepare brownies according to directions. Place aluminum foil in 8 x 8 inch baking pan and coat with cooking spray. Pour half of the brownie batter into the pan. Place graham crackers over brownie mix, then place Hershey bars over graham crackers. Place marshmallows on top of candy bars and pour remaining brownie batter on top. Bake for about 45 minutes or until toothpick inserted into center comes out clean. Let cool completely before cutting (if not you’ll have a gooey mess).