preheat oven to 250°F. Place popcorn in a very large bolw.

in a medium saucepan over medium heat, melt butter. Stir in brown sugar, corn syrup and salt. Bring to a boil, stirring constantly. Boil without stirring 4 minutes. Remove from heat and stir in soda and vanilla. Pour in a thin stream over popcorn, stirring to coat.

Place in two large shallow baking dishes and bake in preheat oven, stirring every 15 minutes, for 1 hour. Remove from oven and let cool completely before breaking into pieces.