Preheat the oven to 250 degrees F (120 degrees C). Spray a large shallow roasting pan with cooking spray, then fill with popcorn.

In a saucepan, melt butter over medium heat. Add brown sugar, corn syrup, and salt. Stir until combined.

Bring to a gentle boil and simmer for 3-5 minutes, without stirring.

Remove from the heat, stir in baking soda and vanilla. The mixture will bubble, and this is normal.

Pour the hot caramel over the popcorn and stir quickly to coat evenly.

Bake for 45 minutes, stirring every 15 minutes for even coating.

Let cool completely. Break into pieces and enjoy!