Preheat the oven to 200 degrees Fahrenheit (93 degrees Celsius).

Line two large baking sheets with a silicone baking mat. NOTE: If only one silicone mat is available, line a second baking sheet with parchment paper (A silicone baking mat yields the best results for this recipe).

Spread the popped popcorn evenly across the prepared baking sheet, and set it aside.

In a saucepan over medium heat, combine the unsalted butter, brown sugar, and corn syrup. Whisk the mixture constantly until the butter has melted and the brown sugar has dissolved. This usually takes about 3-5 minutes.

While continuing to whisk, add the vanilla, salt, and baking soda to the caramel mixture. The baking soda will cause some bubbling.

Remove the saucepan from heat once the caramel has thickened and takes on a lighter, fluffier texture. This typically takes about 1-2 minutes.

Pour the caramel over the popcorn and, working quickly, use a spatula to toss the popcorn, ensuring it is fully coated with caramel.

Place the caramel-coated popcorn in the preheated oven, and bake it for 3 or 4 intervals of 15 minutes each, resulting in a total baking time of 45 minutes. At each 15-minute interval, remove the baking sheet from the oven and toss the popcorn to ensure it is fully coated. Break up any clumps that may have formed. At the 45-minute mark, check if the popcorn is sticky and well-coated.

Transfer the baked caramel popcorn to the second baking sheet to cool. Use a spatula to break up any lumps that may have formed.

For salted caramel popcorn, sprinkle a bit of coarse salt over the popcorn immediately.

Once completely cooled, store the caramel popcorn in an airtight container to maintain its freshness for up to 2-3 weeks.