Preheat the oven to 350 degrees F. Line the bottom and sides of a 9 by 13-inch baking pan with foil. You want to leave about 4 inches of overhang on the 2 opposite sides. These are going to serve as your handles to remove the bars from the pan in 1 piece, so make sure that there is enough of the overhang for you to have a solid grip. Grease the foil well with the softened butter.
In your food processor, grind the graham crackers into a fine meal (you need 2 cups). In a large bowl, combine your crumbs with the sugar, sea salt and melted butter. The mixture should look and feel like wet sand. Set aside a heaping 1/2 cup of the crumb mixture for the topping. Evenly press the remaining crumb mixture into the bottom the foil-lined pan. Pop your crust into the oven and bake until it is golden brown and your kitchen smells of graham crackers, 12 to 15 minutes. Remove the crust from the oven and allow it to cool on a rack or on a dishtowel on your counter.
Melt the milk chocolate in a saucepan over low heat, stirring continuously until fully melted. Do not turn up the heat to hurry this process, you must keep it on low. Alternately, you can microwave the chocolate on low power, stirring after 2 minutes. Once the chocolate is fully melted, it's time to add the chipotle pepper powder, if using, and stir it in completely.
Preheat the broiler.
Pour the melted chocolate over the cooled graham cracker crust. Spread the chocolate evenly over the crust with a butter knife. Sprinkle the marshmallows over the chocolate and press them lightly into the chocolate. Sprinkle the reserved crumb mixture over and in between the marshmallows so that you cannot see the chocolate peaking through the marshmallows. Broil the bars 6 inches from the flame until the marshmallows are golden brown, 1 to 2 minutes.
Refrigerate the bars until the chocolate is hard, 2 hours or so. Grab the ends of the foil liner and lift the bars out of the pan in 1 piece. Move to a cutting board and cut into 24 squares.