Preheat the oven to 250°F (120°C). Line two baking sheets with parchment paper or silicone mats.

Place the popped popcorn in a very large bowl, discarding any unpopped kernels.

In a medium saucepan over medium heat, melt the butter. Stir in the brown sugar, corn syrup, and salt until smooth.

Bring the mixture to a gentle boil, stirring constantly. Allow to boil for 4–5 minutes without stirring until it reaches a light caramel color.

Remove from heat. Quickly stir in the baking soda and vanilla extract; the mixture will foam up.

Pour the hot caramel sauce over the popcorn. Use a silicone spatula to gently toss until all the popcorn is coated.

Divide the coated popcorn between the prepared baking sheets. Bake for 1 hour, stirring and rotating sheets every 15 minutes for even crisping.

Remove from oven and let cool completely on the baking sheets. Break into bite-size clusters before serving.