Preheat oven to 250 F.

Coat bottom and sides of large roasting pan with nonstick cooking spray. Place popped popcorn in roasting pan.

In heavy saucepan, slowly melt butter. Stir in brown sugar, corn syrup and salt. Bring to a boil, stirring constantly. Boil without stirring 5 minutes. Remove from heat. Stir in baking soda and vanilla.

Gradually pour hot mixture over popped popcorn, mixing well.

Bake 1 hour, stirring every 15 minutes. Remove from oven. Cool completely. I usually dump the caramel corn onto sheets of waxed paper on my counter to cool.

Break apart and store in tightly covered container.