Preheat the oven to 350 degrees Fahrenheit. Spray a 9 x 9-inch pan with nonstick cooking spray and line with parchment paper.
Arrange a single layer of 6 sheets graham crackers on the bottom of the pan. Place a layer of 6 (1.55-ounce) bars Hershey's chocolate over the top of the graham crackers.
In a large bowl, prepare 1 box brownie mix according to the package directions.
Pour the batter over the chocolate bars in the prepared pan and bake for 35-40 minutes.
Remove brownies from the oven and top with the 1 (12-ounce) bag large marshmallows. Turn the oven to broil and place the brownies in the oven for 1-2 minutes, until the marshmallows are golden brown and toasted. Let the brownies cool completely before slicing and serving.
Notes
Storage Instructions
Room temperature: Store brownies in an airtight container at room temperature for up to 2–3 days. Keep them covered well so the marshmallows stay soft.
Refrigerator: For longer storage, refrigerate for up to 5 days. Let them come to room temperature before serving for the best texture.
Freezer: Wrap tightly and store in a freezer-safe container for up to 2 months.
To thaw: Let frozen brownies sit at room temperature for a few hours, or warm briefly in the microwave for a gooey, fresh-baked finish.