1Heat the oven to 250°F. Coat a large roasting pan with some butter and place the popcorn in the pan. Set aside.
2Place the brown sugar, butter, corn syrup, and salt in a medium heavy-bottomed saucepan over medium heat and bring to a boil, stirring occasionally with a rubber spatula until the sugar has dissolved, about 5 minutes. Continue to boil without stirring for 3 minutes. Remove from heat, add the vanilla and baking soda, and whisk until the caramel is light in color, foamy, and has doubled in volume, about 10 seconds.
3Immediately drizzle over the popcorn, stir with a rubber spatula until thoroughly coated, and spread into an even layer.
4Bake, stirring every 15 minutes with a rubber spatula and scraping up any caramel from the bottom of the pan, until a cooled piece of popcorn is very crunchy, about 45 minutes to 1 hour total. (To test for doneness, take a few pieces of popcorn out of the oven and let cool for 30 seconds. If they’re crunchy, then the caramel corn is done.)
5Transfer the caramel corn to a work surface or rimmed baking sheet to cool completely (it will crisp as it cools).
6If you want to make your own cheese popcorn. Just pop your popcorn, coat with some butter and toss. Then coat with dry cheese sauce mix (such as the cheese packets you get in your kids kraft macaroni and cheese) and toss again.