Preheat oven to 350 degrees and place rack in the center of the oven.
Line a 13"x9" pan with foil or parchment paper and spray lightly with cooking spray.
Prepare brownie mix or recipe according to instructions.
Place graham crackers in the bottom of the pan, breaking as needed to fit.
Pour half of the brownie batter over the graham crackers and spread very gently with a knife to cover.
Place Oreo cookies in a single layer on top of batter.
Add one cup of mini marshmallows to the remaining brownie batter and fold in gently.
Spread batter gently over Oreos, placing mini marshmallows as evenly as possible.
Bake for approximately 27 minutes or until a toothpick inserted in the middle comes out mostly clean.
Switch to broiler and sprinkle remaining marshmallows evenly 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. *Pro-tip: To prevent sticking, run knife under warm water before each slice.