Preheat the oven to 350°F. Lightly grease a 9x13-inch baking dish with nonstick cooking spray.
In a medium mixing bowl, stir together the graham cracker crumbs, melted butter, and brown sugar until the crumbs are evenly moistened.
Press the crumb mixture firmly into the bottom of the prepared baking dish to form an even crust.
Bake for 5 minutes, or until the edges are lightly golden. Remove from the oven and let the crust cool slightly.
Arrange the Hershey's chocolate bars over the warm crust, covering as much of the surface as possible.
Return the pan to the oven for 2–3 minutes, or until the chocolate is soft and glossy.
Remove from the oven and let the chocolate sit for 1–2 minutes, then gently spread it into an even layer if needed.
Evenly sprinkle the mini marshmallows over the melted chocolate, covering the entire surface.
Set the oven to Broil. Place the pan under the broiler for 1–2 minutes, watching closely, until the marshmallows are lightly toasted and golden brown.
Remove from the oven and allow the bars to cool completely before slicing. For cleaner cuts, run a sharp knife under hot water, dry it well, and slice immediately.