Prepare the Popcorn
Pop enough popcorn to make about 10 cups. Remove all unpopped kernels and spread the popcorn evenly over a large baking sheet or two baking sheets.
Preheat the Oven
Preheat your oven to 250°F (120°C). Lightly grease your baking sheet or line it with parchment paper to make cleanup easier.
Make the Caramel
Add the butter, brown sugar, corn syrup, and salt to a medium saucepan. Heat over medium heat, stirring occasionally, until the butter melts and the mixture comes to a gentle boil.
Cook the Caramel
Once boiling, continue cooking the caramel mixture for about 4 minutes. Stir gently and watch it carefully to prevent it from becoming too dark.
Add Vanilla and Baking Soda
Remove the saucepan from the heat. Carefully stir in the vanilla extract and baking soda. The mixture will bubble and become slightly foamy.
Coat the Popcorn
Immediately pour the warm caramel over the popcorn. Using a large spatula or wooden spoon, gently toss the popcorn until the caramel is distributed as evenly as possible.
Bake
Spread the caramel-coated popcorn into an even layer. Bake at 250°F (120°C) for about 1 hour, stirring every 15 minutes to help coat the popcorn evenly and prevent large sticky areas.
Cool
Remove the caramel corn from the oven and let it cool completely on the baking sheet. Once cool, break apart any large clusters and serve.