Preheat oven to 250 degrees F. Use an air popper to pop 18-20 cups popcorn.

Melt butter in a medium saucepan. Add brown sugar, corn syrup and salt, and stir constantly over medium heat. Once the mixture boils, stop stirring and set a timer for 4 minutes.

Remove from heat and add in baking soda and vanilla. Stir well and carefully pour caramel over popcorn. Combine and split into two pans. (Note: I use disposable foil roasting pans for easier cleanup.)

Place in preheated oven for 1 hour, stirring every 15 minutes. Let cool, then break into individual pieces and store in an air-right container.