Preheat an oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch baking dish with cooking spray.
Combine the graham cracker crumbs, melted butter, sugar, and cinnamon in a mixing bowl. Mix until evenly moistened, then press into the bottom of the prepared baking dish. Bake in the preheated oven until the crust is lightly browned and smells toasted, about 5 minutes.
Place the chocolate into a medium bowl. Heat the cream in a small sauce pan over medium heat. Bring just to a boil. When the cream has come to a boil, remove from the heat immediately and pour over the chocolate chips; whisk until smooth. Stir in the optional rum. Pour evenly over the graham cracker layer.
Preheat the oven's broiler and set the oven rack about 6 inches from the heat source.
Place an even layer of marshmallows on top of the chocolate. Broil until the marshmallows are lightly toasted, about 1 minute. Refrigerate until chocolate has hardened, about 45 minutes. Cut into bars using a pizza cutter sprayed with non-stick cooking spray.
