1. Preheat oven to 250°F (120°C). Line a baking sheet with parchment paper.
2. Pop the popcorn kernels in a large pot according to package instructions. Transfer to a large bowl.
3. In a medium saucepan, combine brown sugar, corn syrup, butter, and salt. Cook over medium heat, stirring constantly, until mixture comes to a boil. Boil without stirring for 4-5 minutes.
4. Remove from heat and stir in baking soda and vanilla extract. The mixture will bubble up.
5. Pour caramel sauce over popcorn and stir to coat evenly.
6. Spread popcorn evenly on the prepared baking sheet. Bake for 45 minutes, stirring every 15 minutes.
7. Remove from oven and let cool completely before breaking into pieces.