Preheat the oven to 200 degrees F.
Line a jelly roll pan with parchment paper. Add popcorn to a large pan, spread out in an even layer.
Melt butter in a large non-stick skillet over medium heat. Once melted, stir in brown sugar, corn syrup, and salt until smooth.
Bring to a boil over medium heat, stirring occasionally. As soon as the caramel starts to gently boil, boil for 2 minutes, stirring constantly.
Remove from the heat and stir in baking soda and vanilla.
Immediately drizzle caramel over popcorn. Toss gently with a spatula until the popcorn is evenly coated.
Bake at 200 degrees F for 45 minutes, stirring the popcorn every 15 minutes.
