Here's what you need..
1 box brownie mix
2 cups graham cracker crumbles
2 sticks butter
20 large marshmallows
First melt the butter in a 9x13 dish in the oven under really high heat (about 450* or so). Add crumbs and press together to form crust.
Make brownie mix, then pour over crust.
Bake according to the directions on the brownie box. Check brownies with a toothpick until the toothpick comes out clean. Once they're done, add marshmallows. Set under broiler for 1-2 min. I pressed them down and set under the broiler one more time. I also think using a ton of mini marshmallows would have looked super cute on this!
Let cool for a couple hours before cutting and serving or else the graham cracker crust wont be ready!