In a medium saucepan add the brown sugar, butter, corn syrup, and salt. Heat on medium and bring to a boil.
Boil one minute constantly stirring. Remove the pot from the heat and stir in the baking soda and vanilla extract.
Pour the caramel syrup over the popcorn evenly and toss with a spatula until coated evenly.
Place the trays into the oven for 1 hour tossing every 15 minutes until the caramel coating has almost dried completely.
You can check for doneness by removing the tray from the oven and allowing it to cool for about 5 minutes after the first hour.
The caramel coating will no longer be sticky when the caramel corn is done.