Prepare brownie mix according to the directions on your box
Line a 9 x 13 glass baking dish with parchment paper
Lay graham crackers all along the bottom of the dish
Using a spatula, spread a layer of the brownie mix over the graham crackers - covering as much of the crackers as possible
Place a layer of Oreo cookies on top of the brownie layer
Layer the marshmallows over the top of the cookies (I used the flat s'more marshmallows - but I cut them each in half so there was a thin layer of marshmallow
Use the remaining brownie batter to cover the marshmallows and along the sides of the Oreo cookies - spread it as evenly as possible.
Bake for 45 minutes (this may vary depending on your oven - check around 35 minutes to see if they are done. Personally, I prefer them a little overcooked than undercooked).
Take out of oven and place a layer of marshmallow on top of the brownies (again, I cut my in half). Place them under the broiler until they start to get golden brown - NOTE: Do not take your eyes off of them, they burn really, really fast. The longest you will need to leave them is 2 minutes, but probably less.
Let them cool completely before cutting - otherwise you will have a huge mess on your hands! I left mine for about 2 hours before I cut them!
IMPORTANT NOTE: Do not take your eyes off of them, they burn really, really fast. The longest you will need to leave them is 2 minutes, but probably less.TIP: To cut your s'more brownies let them cool completely and spray a knife with cooking spray and cut.