Butter a 9-by-13-inch baking dish. Line with parchment, leaving a 2-inch overhang on two sides; butter parchment. Preheat broiler. Butter a rimmed baking sheet; arrange marshmallows on it in an even layer. Broil until blackened in places, 1 to 2 minutes.

Melt butter in a saucepan over medium heat. Add marshmallows; stir until melted. Stir in cereal, graham crackers, and salt. Remove from heat; let cool 1 minute. Stir in half of chocolate. Press mixture into prepared dish; press remaining chocolate into top. Let cool completely. Lift out of pan using parchment; cut into shapes with a sharp knife or cookie cutter.