How to Make It
Butter a 9-by-13-inch baking dish. Pop the popcorn according to the package directions.
Melt the butter and marshmallows in a large saucepan over medium-low heat, stirring until smooth, 5 to 7 minutes.
Fold in the popcorn until coated. Press into the prepared dish and let sit for 10 minutes before cutting into 24 bars.