Preheat oven to 325°F. Spray 6 3.5-inch mini cast iron skillets with nonstick cooking spray, or line an 8×8-inch baking pan with foil and spray the foil with nonstick cooking spray.
In a medium bowl, stir together the graham cracker crumbs, sugar, and butter until combined.
Evenly divide the crumbs between the mini cast iron skillets, or spoon all the crumbs into the prepared 8×8-inch pan. Evenly press the crumbs into the bottom on the skillets or pan to create a crust layer
Bake crust(s) in preheated oven for 8 minutes. Remove skillets or pan from oven and allow them to cool while you make the brownie batter.
Prepare brownie mix according to package directions.
Evenly divide the brownie batter between the skillets, or spoon it into the 8×8-inch pan.
Bake mini skillets for 25 minutes, or the 8×8-inch pan for 40 minutes.
Remove brownies from oven and top with marshmallow halves, cut side down.
Toast marshmallows using your broiler, or use a kitchen torch. Be sure to keep your eyes on the marshmallows because they can go from perfectly toasted to burnt within seconds.
Serve warm. If desired, top with chocolate or caramel sauces.