Heat the oven to 250°F (120°C). Line one or two large rimmed baking sheets or roasting pans with parchment paper.

Place the popped popcorn in a large roasting pan and remove every unpopped kernel.

Melt the butter in a saucepan over medium heat. Add the brown sugar, corn syrup, and salt, then bring the mixture to a boil.

Once boiling, cook the caramel for about 4 to 5 minutes without vigorous stirring, allowing the sugar to dissolve and the mixture to thicken.

Remove the saucepan from the heat and carefully stir in the vanilla and baking soda. The caramel will foam, so add these ingredients slowly and keep your face and hands away from the steam.

Pour the hot caramel over the popcorn and toss thoroughly to coat as evenly as possible.

Spread the coated popcorn in an even layer on the prepared pans. Bake for about 1 hour, stirring every 15 minutes.

Cool the caramel corn completely before breaking it into clusters and serving. The coating hardens as it cools.