Preheat oven to 250 degrees. Line two large, shallow baking dishes with foil and spray with non-stick cooking spray. Place popped popcorn in a very large bowl.

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 popcorn in prepared pans and bake in preheated oven, stirring every 15 minutes, for 30-45 minutes (less if you like chewy popcorn, more if you like it crunchy).

Remove from oven and let cool completely before breaking into pieces.