1. Coat a 15 x 11 x 1-inch jelly roll pan with nonstick cooking spray.
2. In a medium heavybottomed stainless steel saucepan, combine brown sugar, corn syrup, butter, vinegar and salt. Cook over medium-high heat, without stirring, until mixture reaches 300 degrees on a candy thermometer, about 20 minutes.
3. Meanwhile, microwave popcorn following package directions and spread out on prepared jelly roll pan. Carefully pour hot sugar mixture over popcorn and quickly toss with 2 forks until popcorn is coated. Cool.
4. Break into bite-size pieces and store in airtight containers at room temperature.