10 cups popped popcorn, remove and discard all unpopped kernels
1½ sticks (12 tablespoons) butter
1 cup brown sugar
¼ cup + 2 tablespoons corn syrup
½ teaspoon vanilla
¼ teaspoon baking soda
Pinch Kosher salt
Preheat your oven to 250˚F and line a baking sheet with a nonslip baking mat, like a Silpat.
Place the popped corn in a large mixing bowl. In a medium pot, combine the butter, brown sugar, corn syrup and vanilla. Heat over medium heat until the sugar melts, allow the mixture to come to a boil and cook for 3-4 minutes. Remove from the heat and carefully whisk in the baking soda and salt. The mixture will foam and bubble. Drizzle the mixture evenly over the popcorn and stir well until evenly coated. Transfer the popcorn to the prepared baking sheet and bake for 30 minutes, stirring every 10 minutes.
Allow to cool on the baking sheet before storing.