Preheat the oven to 350°F. Prepare the graham cracker crust by mixing them together the crumbs, butter and sugar.
Place a sheet of parchment paper in the bottom of a 9 x 9 square brownie pan. Press the graham cracker crumbs into the bottom of the pan on top of the parchment.
Bake the graham cracker crust for 12 to 14 minutes until lightly browned. Remove the pan from the oven and allow to cool for a few minutes while preparing the brownie mix according to the package directions. (You'll most likely need eggs and oil to prepare the brownie batter.)
Pour the brownie batter on top of the graham cracker crust and return the pan to the oven for 40 to 45 minutes or until the brownies have finished cooking.
Remove the brownies and add enough mini marshmallows on top to completely cover the brownies.
Switch your oven to the broil function. Place the brownie pan in the oven with the door slightly ajar. The marshmallows will begin to brown very quickly, within a minute, so stay close to the oven and remove the pan from the stove to prevent burning.
Allow the brownies to cool completely. Spray a sharp knife with cooking spray before slicing the brownies into bars.