Prepare the Popcorn:
Pop the popcorn kernels in your popcorn maker, air popper, or on the stovetop. You need about 10 cups of popped popcorn. Spread the popcorn out on a large baking sheet lined with parchment paper or a silicone baking mat, removing any unpopped kernels.
Make the Caramel Sauce:
In a medium saucepan over medium heat, melt the butter. Once melted, add the brown sugar, corn syrup, and salt. Stir well and bring the mixture to a boil, making sure to stir constantly to prevent burning.
Once the mixture reaches a boil, stop stirring and let it cook for 4-5 minutes without stirring. It should bubble up slightly and thicken.
Remove from heat and carefully stir in the vanilla extract and baking soda. The caramel will bubble up, so be cautious. Stir until fully combined.
Coat the Popcorn:
Pour the hot caramel sauce evenly over the popped popcorn. Use a spatula or spoon to gently stir the popcorn to coat every kernel with the caramel sauce. Be careful as the caramel will be hot.
Bake the Caramel Popcorn:
Preheat your oven to 250°F (120°C). Once the popcorn is well coated, transfer the baking sheet to the oven. Bake for 45 minutes, stirring every 15 minutes to ensure the caramel coats the popcorn evenly.
After 45 minutes, remove the popcorn from the oven and let it cool for 5-10 minutes.
Cool and Enjoy:
Once cooled, break the popcorn into pieces and enjoy! You can store the caramel popcorn in an airtight container for up to a week.