Preheat oven to 350 degrees. Prepare brownie mix according to package directions. (I'm a brownie purist, I'll be making my own instead of using a mix)
Line foil into an 8x8 inch baking pan and generously spray with cooking spray. Pour half the brownie batter into lined pan.
Layer graham crackers over brownies following with Hershey bars
And 16 large marshmallows.
Pour remaining batter over the marshmallows.
Bake for 40-45 minutes or until toothpick comes mostly clean from center. Remove from oven and let cool completely.
Carefully remove foil from edges and cut into 16 squares.