Preheat the oven to 325 degrees F (170 degrees C) with a rack 8 inches from the heat source. Line a large rimmed baking sheet with aluminum foil. Break graham crackers in half to make squares and arrange in a single layer on the prepared baking sheet. Set aside.
Combine butter and brown sugar in a small saucepan and cook over medium heat, stirring constantly, until melted and smooth, about 3 minutes. Once bubbling, cook, stirring constantly, until starting to thicken, about 3 minutes.
Pour slowly and evenly over the crackers, making sure all of the crackers are covered in the sugar mixture.
Bake in the preheated oven until bubbly and golden, about 15 minutes. Let stand until set, about 30 minutes. Increase oven temperature to broil.
Remove half of the squares and place them on a separate plate. Break each chocolate bar into 4 pieces and place 1 piece on each remaining square; top with 1 marshmallow.
Return to oven and broil until marshmallows are toasted and charred in spots, about 1 1/2 minutes. Remove from oven and top with reserved graham cracker toffee squares. Serve immediately. Recipe developed by Julia Levy Freeman
