Melt butter in a large saucepan over medium heat; stir in salt and marshmallows. Reduce heat to low; cook for 7 minutes or until the marshmallows melt and the mixture is smooth, stirring frequently.
Pour marshmallow mixture over popcorn mixture, stirring to coat well. Lightly coat hands with cooking spray or water; shape popcorn mixture into 20 (2-inch) balls.
Allow mixture to cool enough for kids to handle if they are helping. Popcorn ball will keep for about 3 days when stored in an air-tight container.
