Preheat the oven to 400 degrees F. Spray a baking sheet lightly with nonstick cooking spray.
Place the graham crackers on the baking sheet so they are touching each other. Top with the chocolate and marshmallows. Bake, turning the baking sheet once about halfway through the cooking time, until the marshmallows are puffed and golden all over, about 15 minutes.
Remove and let cool for 5 minutes, then gently push down on the marshmallows to help the chocolate adhere to the graham crackers. Let cool completely, then break the s'mores into pieces. The s'mores can be stored in an airtight container with parchment paper between the layers for up to 5 days.
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