Spray large shallow roasting pan with cooking spray. Add popcorn and place in 250°F oven while preparing glaze.
Combine brown sugar, corn syrup, butter and salt in a heavy 2-quart saucepan.
Stirring constantly, bring to a boil over medium heat. Boil 5 minutes WITHOUT STIRRING. Remove from heat. Stir in baking soda and vanilla and mix well. (Mixture will foam) Pour syrup over warm popcorn mixture, stirring to coat well.
Bake for 45 minutes, stirring occasionally. Remove from oven and spread on foil that has been sprayed with cooking spray.
Cool; break apart. Store in tightly covered container.
Recipe variation: add 1 cup peanuts.