Melt butter in large saucepan on low heat. Add marshmallows, vanilla and salt; cook until marshmallows are completely melted and mixture is well blended, stirring constantly.
Place popcorn in large bowl. Add marshmallow mixture; mix lightly to coat.
Add a small handful of your favorite candy and mix it with a handful of popcorn. It should make about 10 (3-4 inch) balls. The trick is to lightly grease your hands before forming the popcorn. Place on sheets of waxed paper; let stand until firm.