In a large saucepan over medium heat, melt butter, and stir in brown sugar and salt. Stirring frequently, bring to a boil, and cook for one minute.
Remove from heat, and stir in vanilla until fully incorporated. Working quickly, add marshmallows, and mix until almost completely melted. Pour over popped corn, and stir to coat evenly. Spread on parchment-lined counter, and allow to cool completely. For popcorn balls, as soon as it’s cool enough to handle, butter your hands, and form into balls. Allow them to cool completely, and then wrap them individually.