Preheat your oven to 350 degrees F and grease a mini muffin tin with non-stick cooking spray.
In a medium mixing bowl, combine graham cracker crumbs, melted butter, and sugar.
Add roughly 1 tbsp of the graham cracker mixture into the bottom of each muffin tin cavity, then use the bottom of a shot glass to press the graham cracker crumbs into mini crusts.
Prepare your brownie batter following the directions on the box, then evenly divide the batter among each muffin tin cup. Bake for 20 minutes, then set aside to cool.
In the meantime, turn your oven to broil, and cut each marshmallow in half. Top each brownie bite with half of a marshmallow, cut side down, then broil for 30 seconds, watching very closely so they don’t burn!
Immediately top each toasted marshmallow with a square of chocolate, then let cool before removing from the muffin tin and enjoying!