Preheat your oven to 250°F (120°C). Lightly grease a large baking sheet.
Place the popped popcorn in a large bowl, ensuring there are no unpopped kernels.
In a medium saucepan over medium heat, combine the brown sugar, butter, corn syrup, and salt. Stir continuously until the mixture comes to a boil, then let it boil without stirring for 4-5 minutes.
Remove the saucepan from the heat and stir in the baking soda and vanilla extract until well combined.
Quickly pour the caramel sauce over the popcorn and gently stir with a spatula to coat all the popcorn evenly.
Spread the caramel-coated popcorn onto the prepared baking sheet, and bake in the preheated oven for 45 minutes, stirring every 15 minutes.
Remove the caramel popcorn from the oven and let it cool completely before breaking it into clusters. Store in an airtight container.