Preheat oven to 325 degrees F. Line an 8x8 baking pan with foil and spray foil with non-stick cooking spray.
Make the Graham Base: In a medium bowl, stir together the graham cracker crumbs, brown sugar, salt and melted butter until all of the crumbs are evenly moistened. Turn out into the prepared baking pan and press evenly into a tight, compact layer. Bake until it is lightly browned and has a faint toasted smell, about 10 minutes. Remove from the oven.
Make the Chocolate: Place the chocolate and sweetened condensed milk in a heatproof bowl set over a small pan of barely simmering water. Stir occasionally until it is completely melted and smooth. Immediately pour it over the graham cracker crust and spread into an even layer.
Immediately place the marshmallows on top of the chocolate layer, pressing down slightly to nestle them into the chocolate. When ready to serve, turn on your oven's broiler and place the pan on the center rack. Broil until the marshmallows are puffed and browned, 1 to 2 minutes. Remove from the oven and let cool briefly, then slice and serve (I used a large pizza cutter sprayed with non-stick cooking spray to slice mine). Leftovers can be stored in an airtight container for up to 4 days.