Preheat the oven to 300 degrees F. Line a rimmed baking sheet with foil and spray with nonstick spray.
Melt the butter in a 1 gallon pot over medium-high heat. Add the brown sugar and corn syrup. Stir to combine. Bring to a boil, then reduce the heat to low and let simmer undisturbed for 4 minutes. Stir, then continue to cook for an additional 4 to 6 minutes, stirring every 30 seconds, until the mixture is a deep golden color. Turn off the heat and whisk in the salt, baking soda and vanilla. Set the whisk aside and stir in the popcorn. Scoop up the syrup from the bottom and over the top. Keep stirring and scooping the syrup over the popcorn until it is evenly covered and there is no syrup left on the bottom of the pan.
Spread the mixture onto the prepared baking sheet and bake for 15 to 20 minutes, stirring every 5 minutes, until the caramel corn has turned a deep amber color. Remove the caramel corn from the oven and spoon it onto a waxed paper-lined surface to cool. When completely cooled, store caramel corn in a covered container- it should stay fresh for up to a week.
Notes
*You can use air-popped popcorn for this recipe or stovetop too. Here's an easy stove-top method to follow. Microwave popcorn will work as well, but it's not recommended as being the best choice.