Preheat oven to 250°F (120°C). Place popcorn in a large bowl.
In a medium saucepan, combine brown sugar, corn syrup, butter, and salt. Bring to a boil over medium heat, stirring constantly. Boil without stirring for 4-5 minutes.
Remove from heat and stir in baking soda and vanilla extract. Pour over popcorn, stirring to coat evenly.
Spread popcorn onto a baking sheet. Bake for 45 minutes, stirring every 15 minutes.
Remove from oven and let cool completely before breaking into pieces.