Preheat oven to 300 degrees F. Remove all unpopped kernels from the popped corn. Place popcorn in a large roasting pan. In a small saucepan combine corn syrup and butter; cook and stir until butter is melted. Pour corn syrup mixture over popcorn mixture; stir gently to coat. Sprinkle lightly with salt; toss gently to coat.
Bake for 30 minutes, stirring every 10 minutes. Cool mixture in the roasting pan. Break up any clusters before serving. Store leftovers in a tightly covered container at room temperature for up to 1 week.