Prepare the Baking Dish:
Preheat your oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper, leaving overhang for easy removal.
Make the Graham Cracker Crust:
In a mixing bowl, combine graham cracker crumbs, melted butter, and sugar. Mix until the crumbs are evenly coated and resemble wet sand.
Press the mixture firmly into the prepared baking dish, creating an even layer.
Bake the Crust:
Bake the crust for 8–10 minutes, or until lightly golden. Remove from the oven and let it cool slightly.
Add the Chocolate Layer:
Sprinkle the chocolate chips evenly over the warm crust. Let them sit for a minute or two to melt slightly, then spread into an even layer with a spatula.
Top with Marshmallows:
Sprinkle the mini marshmallows evenly over the chocolate layer.
Broil to Toast the Marshmallows:
Turn on the oven broiler and place the dish on the middle rack. Broil for 1–2 minutes, watching closely, until the marshmallows are golden brown and toasted.
Cool and Slice:
Let the bars cool completely at room temperature. For easier slicing, chill them in the refrigerator for 15–20 minutes. Use the parchment overhang to lift the bars out of the dish and cut into squares.