Preheat oven to 300 degrees F. Line a rimmed baking sheet or roasting pan with aluminum foil, and grease with cooking spray.
Pop your popcorn following package directions.
Melt butter in a large stockpot pot over medium-high heat. Add brown sugar and corn syrup, stir to combine. Bring to a rolling boil, then reduce the heat to low and let simmer undisturbed for 5 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 slowly whisk in salt, baking soda and vanilla, being very careful as the mixture will bubble. Stir in the popcorn and toss until it is evenly coated with the caramel, being sure to scrape the caramel off the bottom of the pot.
Spread the mixture onto the prepared baking sheet or pan in an even layer. Bake for 15 to 20 minutes, stirring to toss every 5 minutes, until the caramel corn has turned a deep amber color. Remove the caramel corn from the oven and transfer to a surface covered with parchment or wax paper to cool. Once it reaches room temperature, use your hands or a spoon to break up the caramel corn, then store in an airtight container for up to a week.