Put popcorn in the baking pan.
In a saucepan, mix brown sugar, butter, corn syrup, and salt. Stir. When the mixture comes to a boil, cook for five minutes. Remove from heat.
Stir in vanilla and baking soda.
Cover popcorn with mixture and mix to help coat the popcorn evenly.
Bake 15 minutes at 300°.
Remove, mix again and cook for 5-10 minutes more.
Remove the pan from the oven and let the mixture cool slightly, then break apart into pieces.