Prep: Preheat oven to 250°F. Line a large baking sheet with parchment paper. Place popcorn in a large mixing bowl.
Make caramel: In a medium saucepan, combine brown sugar, corn syrup, butter, and salt. Bring to a gentle boil over medium heat, stirring often. Boil 4–5 minutes without stirring once it bubbles.
Finish the sauce: Remove from heat. Stir in baking soda and vanilla (it will bubble up—this is normal!).
Coat popcorn: Immediately pour caramel over popcorn and stir quickly to coat.
Bake for crunch: Spread coated popcorn onto prepared pan. Bake 45–60 minutes, stirring every 15 minutes.
Cool & break apart: Let cool completely, then break into clusters. Try not to eat half before packaging. 😉