Cover table top with wax paper and spray it with cooking spray. Preheat oven to 250 degrees Spray large roaster with non-stick cooking spray. Pour puffcorn into roaster. Set aside. In a 2 quart sauce pan, melt butter, brown sugar, and corn syrup together over medium heat and stir in salt. Bring to a boil and cook for two minutes. Add vanilla and baking soda (the mixture will foam up). Stir well and remove from heat. Pour over puffcorn and stir until coated. Bake, stirring every 10 minutes with a wooden spoon.Make sure to stir it well so that most of the mixture from the bottom gets moved to the top so it doesn't burn. Most of the recipes I have seen say to bake this anywhere from 40-60 minutes. I think this is a personal preference. The longer you bake it, the darker brown it gets and it starts to taste more like the Karmel Corn you buy at the mall. My family prefers it after 20-30 minutes, on the lighter side. You'll know it's done because the caramel on the spoon will harden after stirring rather than being sticky or gooey. When it's baked to your liking, spread the caramel puffcorn over the waxed paper and cool. If you give it a good stir every now and again as it cools, it won't clump as much or stick to the paper.