Line 2 large rimmed pans with cooking parchment paper. Divide popcorn, granola bar pieces and marshmallows between pans. Spread evenly.
In small microwavable bowl, microwave chocolate chips uncovered on High 1 minute; stir. Heat another 15 to 30 seconds on High; stir until smooth and fully melted. Spoon and drizzle melted chocolate evenly over popcorn mixture.
Refrigerate mixture on pans 20 to 30 minutes or until chocolate is set. Break into pieces; toss to combine.