Line a large cookie sheet (15x21-inch) or two standard sheets with parchment paper or silicone baking mats. Preheat oven to 225°F. Spray a large, non-metal mixing bowl with nonstick spray. (Avoid metal bowls as they can give the popcorn a metallic taste.) Sift baking soda to prevent clumps.
Pop the popcorn and remove any unpopped kernels. Measure 10 cups and transfer to the greased bowl.
In a medium saucepan, combine butter (cut into tablespoon-sized pieces), corn syrup, brown sugar, cream of tartar, and salt. Stir over medium heat until the butter melts and the mixture is smooth. Bring to a full rolling boil, then stop stirring. Let the mixture boil for 5 minutes or until it reaches 235°F on a candy thermometer.
Remove from heat and stir in baking soda and vanilla extract until combined. The mixture will foam up. Quickly pour it over the popcorn and toss with a spatula, stirring for about 2 minutes until evenly coated.
Spread the coated popcorn in a single layer on the prepared cookie sheet(s). Bake for 45 minutes, stirring every 15 minutes. If using two sheets, rotate them between racks.
Remove from the oven and let the popcorn cool completely at room temperature for about 1 hour. The caramel will harden as it cools. Break into clusters and enjoy!
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Notes
Note 1: Use pre-popped popcorn for convenience. Otherwise, pop plain kernels yourself using a stovetop, popcorn machine, or plain (unbuttered and unsalted or lightly salted) packets.Note 2: If your popcorn is already salted, reduce or omit the added salt.Note 3: Cream of tartar helps prevent crystallization in the caramel. It’s optional but recommended for a smoother caramel.Storage: To store Caramel Corn, place in an airtight container. I like using Mason jars or Weck® mold jars. Keep the container in a cool dry place for up to a week.