1. Preheat oven to 275 degrees F. Sprinkle 1/4 tsp. of the salt over popcorn in large bowl; toss to coat. Set aside.
2. Melt chocolate and butter in large saucepan on low heat, stirring frequently. Add marshmallows; cook until completely melted, stirring frequently. Remove from heat; stir in remaining 1/4 tsp. salt. Pour over popcorn; toss to coat. Spread onto greased baking sheet.
3. Bake 20 to 25 minutes or until crisp, stirring after 15 minutes. Cool on wax paper.