Prepare popcorn according to package directions. Remove all unpopped kernels. Place popped corn in large bowl. Spray 13x9-inch glass baking dish with cooking spray; set aside.
Melt butter in medium saucepan over medium heat. Add marshmallows. Heat 5 minutes or until marshmallows melt, stirring frequently. Add peanut butter and peanuts; stir until well combined. Pour over popcorn; mix well with spoon sprayed with cooking spray.
Place popcorn mixture in glass dish and press evenly using back of spoon. Sprinkle chocolate morsels evenly over popcorn mixture. Microwave on HIGH 1 minute or until chocolate begins to melt. Refrigerate 1 hour or until chocolate is set.
Cut into 18 squares with serrated knife. Store in tightly sealed container up to 3 days.