Prep the popcorn. Heat oven to 250°F. Line a large baking sheet (or two) with parchment paper. Spread the popped popcorn out in an even layer and remove any unpopped kernels.

Make the caramel. In a medium saucepan, melt the butter over medium heat. Add the brown sugar, corn syrup, and salt. Stir until combined, then bring the mixture to a gentle simmer. Once bubbling, cook without stirring for 4–5 minutes until the caramel is smooth and slightly thickened.

Finish the caramel. Remove from heat and immediately stir in the vanilla and baking soda. (The mixture will foam up — this is normal and helps create crisp caramel corn!)

Coat the popcorn. Quickly pour the caramel evenly over the popcorn. Use a silicone spatula to gently toss until as evenly coated as possible.

Bake low and slow. Transfer the popcorn to the oven and bake for 45 minutes, stirring every 15 minutes to help it crisp and coat evenly.

Cool and break apart. Remove from the oven and sprinkle with flaky sea salt if you’d like. Let the caramel corn cool completely, then break into pieces.

Serve or store. Enjoy right away or store in an airtight container for up to 1–2 weeks.