Old Bay seasoning is our secret ingredient. It's the perfect savory balance for the sweet caramel.
Place popcorn in a large bowl; lightly coat with cooking spray. Sprinkle with Old Bay seasoning, and toss to coat.
Melt butter in a 2-qt. heavy saucepan over medium heat. Stir in sugar and vinegar, and bring to a boil. Boil, without stirring, 8 to 10 minutes or until a candy thermometer registers 300°. Immediately pour butter mixture over popcorn mixture, and stir to coat. Spread popcorn on wax paper, and cool completely (about 25 minutes). Break into pieces, and store in an airtight container up to 1 week.
Note: We tested with Newman's Own Natural Tender White Microwave Popcorn.