Preheat the oven to 375 degrees F. Line a 13-by-18-inch rimmed baking sheet with parchment.
Knead the graham cracker crumbs into the cookie dough. Roll out the dough mixture on top of a sheet of parchment to a 12-by-8-inch rectangle. Use a knife to trim and square the sides, then divide into two 6-by-8-inch pieces. Transfer the cookies to the prepared baking sheet, leaving at least 2 inches between them. Bake until golden, 12 to 15 minutes.
While the cookies are still warm, use a skewer to create 2 intersecting dotted lines on top of each cookie. Poke 11 small holes in each of the 4 quadrants created by the intersecting lines to mimic store-bought graham crackers. Cool on the baking sheet, about 30 minutes.
Set aside one of the cookies. Cover the remaining cookie on the baking sheet with the chocolate bars and top with the marshmallows. Brown the marshmallows with a kitchen blowtorch until nicely toasted (or, alternatively, broil on low until the marshmallows are nicely toasted, 2 to 3 minutes). Top with the remaining cookie. Squish and break into large pieces to serve.
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.
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