Preheat oven to 250°F (120°C). Line a large baking sheet with parchment paper.
Place popped popcorn in a large bowl, removing any unpopped kernels.
In a saucepan over medium heat, melt butter. Stir in brown sugar, corn syrup, and salt.
Bring to a boil and cook for 4–5 minutes, stirring occasionally.
Remove from heat and quickly stir in baking soda and vanilla. The mixture will foam.
Pour caramel over popcorn and stir gently until evenly coated.
Spread popcorn onto the prepared baking sheet.
Bake for 1 hour, stirring every 15 minutes for even coating.
Remove from oven and let cool completely before breaking apart and serving.