Preheat oven to 325°.
Combine the first 3 ingredients in a large bowl. Spread mixture onto a jelly-roll pan lined with foil and coated with cooking spray.
Combine brown sugar, corn syrup, and butter in a medium saucepan. Bring to a boil over medium heat, stirring constantly. Cover and cook 1 minute. Uncover and cook, without stirring, until the candy thermometer registers 290° (about 5 minutes).
Drizzle the brown sugar mixture over popcorn mixture; toss to coat. Bake at 325° for 30 minutes, stirring once. Cool completely on a wire rack in pan, and break into large pieces.
Note: Store in an airtight container for up to 2 weeks.