1. Preheat the oven to 250degreesF. Spray a 9-by- 13-inch pan with vegetable cooking spray. Transfer the popcorn to the baking pan and put it in the oven to warm slightly while preparing the caramel.
2. In a medium saucepan, heat butter, brown sugar, and corn syrup over mediumheat, stirring constantly, until butter melts and mixture is smooth. Increase heat to medium-high and bring to a boil. Without stirring, boil for 2 minutes, swirling the pan gently if necessary to cook the caramel evenly. Remove from heat and use a wooden spoon to stir in the baking soda. The caramel will foam slightly. Stir in vanilla.
3. Remove popcorn from oven and slowly pour caramel over it. Stir the mixture to coat the popcorn thoroughly, scraping bottom of pan. Bake 20 minutes and stir well again; caramel will still be soft. Bake another 40 minutes. Caramel corn should be stiff. Remove from the oven and stir caramel corn to loosen it from the bottom of the pan.
4. Cool in the pan, about 45 minutes. The caramel coating will become firm as itcools. Serve or store in a resealable container at room temperature for up to 1 week.
Source: Fresh Magazine September, October 2010