First off, you'll need a box of your favorite brownie mix, along with the required additional items the recipe calls for (vegetable oil, water, and two eggs in this case). For the "s'mores" portion you'll need about half a bag of mini marshmallows, one packet of graham crackers, and a large milk chocolate bar.
Go ahead and prepare the brownie batter according to the back of the box and pour into a 9x13" baking dish. Preheat your oven according to the instructions on the box...but reduce the baking time by about 3-5 minutes.
Once you pull the brownies out of the oven, generously cover the entire surface with your mini marshmallows, milk chocolate pieces, and crumbled graham crackers.
Place the dish back into the oven, and set the broiler for about 3-5 minutes. Each oven is different, so pay careful attention that your brownies aren't getting burnt to a crisp...you just want the chocolate slightly melted and marshmallows lightly browned!
Once complete, remove the dish from the oven and let cool for about 15 minutes...careful, the marshmallows will be HOT! Once cooled, cut into squares and enjoy!