Preheat your oven to 250°F (120°C) and line a large baking sheet with parchment paper.

In a medium saucepan, combine brown sugar, light corn syrup, unsalted butter, and salt. Stir over medium heat until boiling, then allow to boil without stirring for 4-5 minutes.

Remove from heat and stir in vanilla extract and baking soda.

In a large bowl, pour the hot caramel mixture over the plain popped popcorn and toss gently until coated.

Transfer the coated popcorn to the baking sheet and spread into an even layer.

Bake for 1 hour, stirring every 15 minutes until it looks glossy and crisp.

Let cool completely before breaking apart and transferring to an airtight container.