Place popped corn in a large greased roasting pan to keep warm in a 300 degree oven. (I pop the corn on the stove top in 2 tbsp of coconut oil, but air popped works well too.)
Using two half teaspoon measuring spoons, measure about 1/4 tsp of baking soda and about 1/4 tsp of vanilla and set by the side of the stove so they don't spill.
In a saucepan, stir together brown sugar, butter, and corn syrup on medium heat. Cook and stir to boiling (a steady boil - not just a few bubbles on the edges, but not necessarily a "rolling boil" either) and then cook and stir over medium heat for an additional 4 minutes. Be sure to scrape the bottom often to avoid burning. (For those candy makers out there, you're looking for 255, hard-ball stage.).
Remove saucepan from heat, stir in baking soda and vanilla, and pour over popcorn. Stir gently to coat. Bake in a 300 degree oven for 15 minutes. Stir. Bake 5 minutes more. Pour out popcorn onto a large piece of foil to cool. Cool completely before storing in an airtight container or eat it before its all the way cool.