baking sheets and a rubber spatula or wooden spoon.
Working in batches, place 2 cups popcorn in food processor. Pulse until crushed into small pieces. Do not overprocess. Repeat with remaining popcorn.
Stir sugar, corn syrup, water and salt in a medium saucepan over medium heat until sugar dissolves and mixture is simmering. Clip a candy thermometer to the inside of the pan and continue heating undisturbed until temperature reaches 270F (about 10 to 15 minutes). Mixture will boil vigorously.
Stir in butter, molasses and vanilla. Continue heating until temperature reaches 290F (between soft- and hard-crack stages).
Remove pan from heat; stir in crushed popcorn. Working quickly, pour mixture over prepared sheets. Using the buttered rubber spatula, press evenly to flatten like peanut brittle. Cool to room temperature on a wire rack and break into chunks.