Pop 10 cups of plain popcorn and place it in a large bowl
Preheat oven to 250°F
Line a baking sheet with parchment paper or lightly grease it
In a saucepan, melt 1 cup butter over medium heat
Stir in 2 cups brown sugar, 1/2 cup corn syrup, and 1/2 teaspoon salt
Bring the mixture to a boil, stirring constantly
Boil for 4 to 5 minutes without stirring
Remove from heat and stir in 1/2 teaspoon baking soda and 1 teaspoon vanilla extract
Pour the caramel over the popcorn
Stir until the popcorn is evenly coated
Spread the coated popcorn onto the prepared baking sheet
Bake for 1 hour, stirring every 15 minutes
Remove from the oven and let it cool completely
Break apart the caramel corn and serve