Preheat oven to 250°F (120°C). Line a baking sheet with parchment paper.
Place popped popcorn in a large mixing bowl.
In a saucepan, melt butter over medium heat. Stir in brown sugar, corn syrup, and salt. Bring to a boil, stirring constantly. Boil without stirring for 4-5 minutes.
Remove from heat and stir in baking soda and vanilla extract. The mixture will bubble up.
Pour caramel sauce over popcorn and stir to coat evenly.
Spread popcorn onto the prepared baking sheet. Bake for 45 minutes, stirring every 15 minutes.
Remove from oven and let cool completely before breaking into pieces.