Preheat the oven to 250 degrees F. Line two large, rimmed jelly roll pans with parchment paper. Lightly spray the parchment paper with nonstick cooking spray.
Place the 28 cups popped popcorn evenly across the two prepared jelly roll pans.
Add the cubed 1 cup butter, ½ cup light corn syrup, 2 cups light brown sugar, and 1 teaspoon salt to a medium saucepan. Place over medium heat and bring up to a boil. Let boil for 5 minutes, stirring constantly.
Once the caramel syrup has boiled for 5 minutes, remove from the heat and stir in the ½ teaspoon baking soda and 1 teaspoon vanilla extract. It will foam up.
Pour the syrup evenly over the top of the popcorn. Gently stir to help evenly distribute and thoroughly coat the popcorn.
Place the popcorn in the preheated oven and bake for 1 hour, stirring every 15 minutes.
Remove the popcorn from the oven and allow to cool completely.
Once cooled, in an airtight container at room temperature.
Notes
Storage: Store it in an airtight container or sealed bag at room temperature for up to 1 to 2 weeks.