Preheat the oven to 200°F.
Line a large cookie sheet with parchment paper.
Add popcorn to pan in an even layer and place in the oven to warm
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 light boil over medium heat, stirring occasionally.
As soon as the caramel starts to gently boil, boil for 2 minutes, stirring.
Remove from the heat and stir in baking soda and vanilla.
Pay attention, because it will rapidly expand and will make an enormous mess.
Seriously. Pay attention.
Immediately drizzle caramel over popcorn. Toss gently with a spatula until coated.
Bake for 45 minutes, stirring the popcorn every 15 minutes.