9 popcorn balls
Melt butter in large microwavable bowl on HIGH for 1 minute. Add marshmallows, vanilla and salt. Microwave at HIGH for 1 1/2 minutes until melted and smooth, stirring after 45 seconds. Place popcorn in large bowl; pour marshmallow mixture over popcorn, mixing lightly to coat. With hands lightly greased, shape mixture into 9 (3-inch) balls. Place on waxed paper; let stand until firm.