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1
Heat the oven to 250 degrees F and arrange a rack in the middle.
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2
Coat a large roasting pan with butter and place the popcorn in the pan; set aside.
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3
Place 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.
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4
Continue to boil without stirring for 3 minutes.
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5
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.
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6
Immediately drizzle over the popcorn, stir with a rubber spatula until thoroughly coated, and spread into an even layer.
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7
Bake, 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.
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8
(To test for doneness, take a few pieces of popcorn out of the oven and let cool for 30 seconds.
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9
If theyre crunchy, then the caramel corn is done.)
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10
Transfer the caramel corn to a work surface or rimmed baking sheet to cool completely (it will crisp as it cools).
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11
Store in an airtight container at room temperature for up to 1 week.
