Butter an 8-inch square baking pan. Line bottom and two sides with parchment paper, leaving a 2-inch overhang on both sides. Butter paper; set pan aside.
In a large saucepan, combine butter, marshmallows, and cocoa. Cook over medium, stirring frequently, until melted, about 6 minutes; stir in rice cereal. Press rice mixture into prepared pan; drizzle with melted chocolate. Let cool to room temperature; cut into 16 bars. (To store, keep in an airtight container at room temperature, up to 5 days.)