Pop 1/3 cup unpopped popcorn kernels to yield about 12 cups popped popcorn using a stovetop pot, air popper, or popcorn machine. Spread on a baking sheet or in a large bowl and remove any unpopped kernels.

In a heavy-bottomed saucepan, melt 1 cup unsalted butter over medium heat, then add 1 cup packed brown sugar and 1/4 cup light corn syrup, stirring constantly for about 2 minutes until mixture is smooth.

Increase heat to medium-high, insert a candy or deep-fry thermometer without touching the pan bottom, stop stirring when it boils, and cook until it reaches 280 to 290°F (hardball stage), about 8 to 10 minutes.

Remove saucepan from heat and carefully stir in 1 teaspoon vanilla extract, stirring slowly to avoid splashing.

Immediately add 1/2 teaspoon baking soda and stir vigorously for 30 seconds until foamy, then add 1 teaspoon sea salt and stir to combine.

Quickly drizzle the hot caramel over the popcorn in a thin stream while stirring constantly with a wooden spoon or spatula for 1 to 2 minutes to coat evenly.

Spread the coated popcorn in a single layer on a parchment-lined baking sheet, gently separate any clumps, and let cool at room temperature for 15 to 20 minutes until caramel hardens.