Line a baking sheet with parchment. Heat the chocolate in a medium microwave-safe bowl in the microwave at 75 percent power until soft, about 2 minutes. Stir the chocolate and microwave again until melted, up to 2 minutes more. (Alternatively, put the chocolate in a heatproof bowl. Bring a saucepan filled with an inch or so of water to a very slow simmer; set the bowl over, but not touching, the water, and stir occasionally until melted and smooth.)
Position the oven rack 3 inches below the heating element of the broiler and preheat the oven to broil. Place the marshmallows on a rimmed baking sheet so they are not overlapping, and broil until the tops just begin to toast, about 10 seconds, turning the pan for even browning. (Alternatively, you can use a kitchen torch to char the marshmallows.) Allow to cool at room temperature for 10 minutes.
Fold half the marshmallows and graham crackers into the chocolate, and stir to coat. (Some of the marshmallows may melt, but that's OK.) Pour the chocolate onto the prepared baking sheet, and spread evenly with an offset spatula into an 8 1/2-by-11 inch rectangle. Poke the rest of the marshmallows and the graham cracker pieces into the chocolate in a random yet attractive pattern. (This part may be messy, but stretching the marshmallows into the bark will look cool.) Lightly press down the toppings. Allow the bark to set in the refrigerator for 1 hour. Break into 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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