Line a baking sheet with foil; coat foil with cooking spray. Set aside.
Place popcorn in a large zip-top plastic bag; seal. Crush popcorn using a meat mallet or rolling pin; set aside.
Combine sugar, syrup, and water in a medium saucepan over medium heat. Cook 1 minute or until sugar dissolves, stirring constantly. Cook, without stirring, until candy thermometer registers 270 (about 8 minutes). Stir in molasses and butter; cook until thermometer registers 290 (about 5 minutes). Stir in baking soda, vanilla, and salt. Stir popcorn into boiling syrup mixture. Working quickly, pour popcorn mixture onto prepared pan; spread to 1/4-inch thickness using a wooden spoon coated with cooking spray. Cool completely; break into large pieces.
Note: Store brittle in an airtight container at room temperature for up to four days.
