Spray large shallow roasting pan with cooking spray. I used my butter wrappers instead of spraying. Add popcorn and place in preheated 250 degree oven while preparing caramel.
Mix brown sugar, corn syrup, salt, butter, and salt in a 2-quart saucepan. I skipped the salt because I was using salted popcorn, but your personal preference here. Stirring constantly, bring to a boil over medium heat.
Boil 5 minutes without stirring. Remove from heat. Stir in baking soda and vanilla and mix well.
Pour syrup over warm popcorn and stir evenly to coat.
Bake for 45 minutes, stirring occasionally. Remove from oven and spread on foil that has been sprayed with cooking spray. (I skipped the cooking spray here too).
Cool and break apart. Store in tightly covered container. (I think the purpose of the tightly covered container is so you can't open and eat it as quickly).