Place popcorn in a large bowl. Line a large rimmed baking sheet with foil and spray with non-stick cooking spray.

In a large saucepan over medium heat, melt the butter.

Add brown sugar and corn syrup and cook and stir for 2-3 minutes, until well blended.

Reduce heat to medium low and add the marshmallows. Cook and stir until marshmallows are completely melted. Remove from heat and stir in vanilla.

Pour mixture over popcorn and stir to coat. Spread popcorn onto prepared baking sheet to cool completely.