Preheat your oven to 250°F (120°C). Line a baking sheet with parchment paper.
Pop the popcorn kernels in a large pot. Transfer the popped popcorn to a large bowl, removing any unpopped kernels.
In a medium saucepan, combine the brown sugar, corn syrup, butter, and salt. Bring to a boil over medium heat, stirring constantly. Let it boil for 5 minutes without stirring.
Remove from heat and stir in the baking soda and vanilla extract. The mixture will bubble up.
Pour the caramel sauce over the popcorn and stir well to coat evenly.
Spread the caramel-coated popcorn onto the prepared baking sheet. Bake in the preheated oven for 30 minutes, stirring every 10 minutes.
Remove from the oven and let it cool completely before breaking into pieces.