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 medium saucepan, melt butter over medium heat. Stir in brown sugar, corn syrup, and salt. Bring to a boil, stirring constantly. Let boil for 4–5 minutes without stirring.
Remove from heat and stir in baking soda and vanilla. The mixture will foam.
Pour caramel over popcorn and stir until evenly coated.
Spread coated popcorn on the prepared baking sheet in an even layer.
Bake for 45 minutes, stirring every 15 minutes.
Remove from oven, let cool completely, then break into clusters and serve.