Place popped corn in a shallow baking pan in a 250 degree oven while making the caramel. (I divide the popped corn between two 9x13 pans, glass or metal work fine).
In a medium saucepan, mix together the brown sugar, corn syrup, butter, and salt.
Stirring constantly over medium heat, bring the mixture to a boil. Once it starts boiling (really boiling), stop stirring and set the timer for 5 minutes.
Remove from heat and stir in the vanilla extract and baking soda.
Divide the caramel between the two pans of popped corn and stir it to coat evenly.
Bake at 250°F for a total of one hour, stirring it every 15 minutes.
After one hour, remove the caramel corn from the oven and immediately dump it out onto the countertop (dump onto parchment paper for easy clean-up. Allow the caramel corn to cool, break it apart, and start eating!
Notes
You MUST store the caramel corn in an airtight container (I like to use plastic ziploc bags). It looks cool in a big glass jar, but once the party is over,