Preheat oven to 250 degrees
Place 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 baking soda and vanilla. Pour in a thin stream over popcorn, stirring to coat.
Line a large two large, shallow baking dishes with ALCAN® Non-Stick Baking Foil.
Place caramel popcorn into baking dishes bake in preheated oven, stirring every 15 minutes, for 1 hour. Remove from oven and let cool completely before breaking into pieces.