1. Pop 2 cups popcorn w/ air popper. Put in large bowl or large paper bag.
2. In a saucepan over medium heat, combine the brown sugar, corn syrup, and butter. Cook stirring to melt the butter and dissolve the sugar, until the mixture comes to a boil. Once the mixture is at a full boil, continue to cook for 5 minutes, stirring constantly.
3. Remove from the heat, and stir in the salt, vanilla, and baking soda. The mixture will foam up when you add the soda, so use a long spoon to stir. Immediately pout the sugar mixture over the popcorn in the bowl or bag. Shake or stir until evenly coated.
For a crispier treat, boil the sugar mixture longer, or bake the finished mix in the oven at 350' F for about 10 minutes.