Coat an 8-inch square baking pan with cooking spray.
Combine marshmallows, butter, and salt in a Dutch oven over medium heat. Cook mixture until melted and smooth, stirring constantly. Remove from heat. Stir in popcorn. Transfer mixture to pan, pressing with parchment paper coated with cooking spray.
Place chocolate in a microwave-safe bowl. Microwave at HIGH 1 minute; stir until smooth. Drizzle chocolate over popcorn mixture. Refrigerate 5 minutes. Lift mixture from pan, and cut into 12 bars.