Position a rack in the center of the oven and preheat to 350 degrees F. Line a 9-by-13-inch broiler-proof baking dish with foil, leaving a 2-inch overhang on both sides. Butter the foil.
Pulse 14 of the graham crackers into fine crumbs in a food processor. Combine the melted butter and graham cracker crumbs in a medium bowl with your hands (the mixture should hold together when squeezed). Transfer the mixture to the prepared baking dish and press it into the bottom in an even layer, using the bottom of a measuring cup to help. Pour the sweetened condensed milk over the crumbs. Then sprinkle the chocolate chips, peanut butter chips, and peanuts over the condensed milk. Break the remaining 4 graham crackers into chunks and sprinkle evenly over the top.
Bake until the sides are golden brown and begin to pull away from the baking dish, 30 to 35 minutes. Remove from the oven, and turn the broiler on. Scatter the marshmallows over the top of the bars and put the dish under the broiler until the marshmallows are dark golden and toasted, 3 to 4 minutes. Let cool completely then refrigerate to set, about 1 hour. Using the foil liner as handles, pull the bars out of the baking dish; remove the foil. Cut the bars into 2-inch squares.
To avoid having the marshmallows stick to your knife when you cut up the bars, dip the knife in hot water between cuts and wipe it with a clean towel.
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