Pop popcorn 4 quarts. spray large shallow roasting pan with cooking spray. Add popcorn and place in preheated 250 degrees F oven while preparing caramel.
Mix brown sugar, corn syrup, butter & salt in a 2 quart pan. Stirring constantly, bring to boil over medium heat. Boil 5 minutes no stirring. Remove from heat. Stir in baking soda & vanilla & mix well
Pour syrup over warm popcorn, stirring to coat evenly.
Bake for 45 minutes, stirring occasionally.
Then pour on aluminum foil that has been sprayed with cooking spray. Cool and break apart, store in tightly covered container