Prepare the brownie batter as directed on the box and preheat the oven to 350 degrees.
Spray a 9x13 baking dish with cooking spray.
Line the bottom of the baking dish with graham crackers, breaking them into pieces to fit as needed.
Pour half of the brownie batter over the graham crackers. Top with 1 cup of the mini marshmallows. Break the Hershey's bars into pieces and place half of the pieces on top of the marshmallows.
Top with the remaining graham crackers as evenly as possible. Pour on the remaining brownie batter, top with the rest of the marshmallows and Hershey's bar pieces.
Bake according to package directions or until the brownies are baked through.