Preheat oven to 250°F. Line two rimmed sheet pans with parchment paper or Silpat liners. Have your popcorn popped and at the ready in a large crock or big Dutch-oven (or a collection of your biggest bowls).

In a large, heavy saucepan bring to boil the brown sugar, butter, water, and salt. Continue a moderate boil without stirring for about 5 minutes until mixture starts to thicken.

Technically this is the “soft ball stage”, 234°F on a candy thermometer (less 10° if you are at 5K feet elevation like in Bozeman Montana). If you don’t have a candy thermometer, you can drop a half-teaspoon of the caramel into a glass of cold water to check if it comes together into a blob when you touch it with your finger.

Remove the saucepan from the heat and stir in the vanilla and baking soda. The caramel will bubble and puff up as you do this, be careful as it is very hot and sticky. Pour the caramel over the popped corn kernels and stir gently to coat.

Divide the caramel corn between the two pans and bake for 45 minutes, stirring several times during baking. Allow to cool for about 30 minutes then enjoy!

Keep airtight; it lasts a week or two.