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, combine 1 cup brown sugar, 1/2 cup butter, 1/4 cup corn syrup, and 1/2 teaspoon salt
Bring to a boil over medium heat, stirring constantly
Boil for 4 to 5 minutes without stirring
Remove from heat
Stir in 1/2 teaspoon baking soda and 1 teaspoon vanilla extract
Pour caramel over the popcorn
Stir until evenly coated
Bake for 1 hour, stirring every 15 minutes
Remove from oven and spread on wax paper or parchment paper
Let cool completely
Break into pieces and serve