Preheat your oven to 250°F (120°C). Line a large rimmed baking sheet with parchment paper or a silicone baking mat. Place your popped popcorn in a very large mixing bowl, making sure to remove any unpopped kernels.
In a medium saucepan over medium heat, combine the butter, brown sugar, corn syrup, and salt. Stir the mixture constantly until the butter is completely melted and the sugar has dissolved.
Bring the mixture to a boil. Once it starts boiling, stop stirring and let it bubble for exactly 4 to 5 minutes. This helps the caramel reach the right consistency for a crunchy coating.
Remove the saucepan from the heat. Quickly stir in the vanilla extract and the baking soda. The mixture will foam up and become lighter in color; this is normal and creates a light, airy texture for the caramel.
Immediately pour the hot caramel over the popcorn in the large bowl. Use a large spoon or spatula to toss the popcorn gently until every piece is well-coated. Work quickly before the caramel starts to set.
Spread the coated popcorn onto your prepared baking sheet in an even layer. Bake for 45 minutes, stirring every 15 minutes. This step dries out the caramel so it isn’t sticky and helps it stay crunchy for days.
Remove the pan from the oven and let the popcorn cool completely on the tray. Once cooled and hardened, break the large mass into bite-sized clusters with your hands.