Preheat your oven to 250°F (120°C). Line a baking sheet with parchment paper.
Place the popped popcorn in a large mixing bowl.
In a saucepan, combine the brown sugar, corn syrup, butter, and salt. Cook over medium heat, stirring constantly, until the mixture comes to a boil.
Once boiling, stop stirring and let it boil for 5 minutes.
Remove from heat and stir in the peanut butter, baking soda, and vanilla extract until smooth.
Pour the caramel mixture over the popcorn and stir until evenly coated.
Spread the coated popcorn onto the prepared baking sheet.
Bake in the preheated oven for 45 minutes, stirring every 15 minutes.
Remove from the oven and let it cool completely before breaking into pieces.