PREHEAT oven to 275°F. Sprinkle 1/4 tsp. of the salt over popcorn in large bowl; toss to coat. Set aside.
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.
BAKE 20 to 25 minutes or until crisp, stirring after 15 minutes. Cool on wax paper.