Preheat oven to 250°F
Pop 10 to 12 cups of plain popcorn
Remove any unpopped kernels
Place popcorn in a large greased roasting pan or bowl
In a saucepan, melt 1 cup butter
Add 2 cups brown sugar
Add 1/2 cup corn syrup
Add 1/2 teaspoon salt
Stir over medium heat until combined
Bring to a boil
Boil for 4 to 5 minutes without stirring
Remove from heat
Stir in 1/2 teaspoon baking soda
Stir in 1 teaspoon vanilla extract
Pour caramel over popcorn
Stir until evenly coated
Spread coated popcorn on a baking sheet
Bake for 1 hour, stirring every 15 minutes
Remove from oven
Cool completely
Break apart and serve