Preheat the oven to 350 degrees. Line a 9x9 brownie pan with parchment paper and grease it. Set aside.
Take your graham crackers and lay them out evenly on the bottom of the pan.
In a mixing bowl, make your brownie mix as directed on the box. Pour over the graham crackers.
Place in the oven and bake as directed on the box (30-40 minutes) or until a toothpick inserted comes out clean.
Remove from the oven and sprinkle with the marshmallows. Place back in the oven and turn on the broiler. Watch carefully and let them toast a bit.
Remove from the oven and immediately sprinkle with Peanut M&M'S®. Let them fully cool and then remove from the pan.
To get a clean cut through the marshmallow, pop the brownies in the freezer for 10-15 minutes. Run your knife under hot water and cut quickly. It'll make a clean cut. Then leave the brownies out to come back to room temperature.