Preheat oven to 350 degrees and place rack in the center of the oven.
Line an 8"x8" pan with foil and spray lightly with cooking spray.
Prepare brownie mix according to package instructions.
Place graham crackers in the bottom of the pan, breaking as needed to fit.
Add peppermint baking chips and one cup of mini marshmallows to the brownie batter and fold in gently.
Pour brownie batter over the graham crackers and spread very gently with a knife to cover.
Bake for approx. 27 minutes or until a toothpick inserted in the middle comes out almost clean.
Switch to broiler and sprinkle remaining marshmallows on top of brownies.
Place pan back into the center of the oven and leave door cracked open for about two minutes so marshmallows can brown. Watch closely as they can burn quickly!
Allow to cool completely before cutting into squares.