Preheat Oven : Set oven to 250°F (120°C) and line a baking sheet with parchment paper.
Pop Popcorn : Pop kernels using stovetop or air popper. Transfer to a large mixing bowl, removing any unpopped kernels.
Make Caramel Sauce : In a medium saucepan, melt butter over medium heat. Stir in brown sugar, corn syrup, and salt. Bring to a boil, stirring constantly.
Cook Caramel : Allow the mixture to boil for 4–5 minutes without stirring, ensuring it thickens.
Add Finishing Touches : Remove from heat and quickly stir in baking soda and vanilla extract. The mixture will foam slightly—this is normal.
Coat Popcorn : Pour caramel sauce over popcorn and toss gently with a spatula until evenly coated.
Bake Popcorn : Spread coated popcorn on the prepared baking sheet in an even layer. Bake for 45 minutes at 250°F, stirring every 15 minutes for even coating.
Cool : Remove from oven and let cool completely on the baking sheet. Break into clusters once cooled.