Preheat the oven to 200 degrees Fahrenheit (about 93 degrees Celsius).
Cover two large baking sheets with a silicone baking mat. If you only have one mat, use parchment paper for the second sheet (though a silicone mat works best).
Spread the popped popcorn evenly on the prepared sheets and set it aside.
In a saucepan over medium heat, mix the unsalted butter, brown sugar, and corn syrup. Keep stirring until the butter melts and the brown sugar dissolves, which usually takes 3-5 minutes.
While still stirring, add the vanilla, salt, and baking soda to the caramel mixture. The baking soda will make it bubble a bit.
Remove the saucepan from heat once the caramel thickens and gets a lighter, fluffier texture, typically in 1-2 minutes.
Quickly pour the caramel over the popcorn and use a spatula to toss it, making sure every piece is coated.
Put the caramel-coated popcorn in the preheated oven. Bake it in 15-minute intervals, tossing it each time, for a total of 45 minutes. Check at the 45-minute mark to ensure it’s sticky and well-coated.
Transfer the baked caramel popcorn to the second baking sheet to cool. Use a spatula to break up any clumps.
For salted caramel popcorn, sprinkle some coarse salt immediately.
Once completely cooled, store the caramel popcorn in an airtight container to keep it fresh for 2-3 weeks. Enjoy!