Place popcorn in a large bowl. In a microwave-safe bowl, combine the butter, sugar and cinnamon. Microwave on high for 1 minute; stir. Microwave 1 minute longer or until the butter is melted. Pour over popcorn and toss to coat. Transfer to two greased 15-in. x 10-in. x 1-in. baking pans. Bake, uncovered, at 300 degrees F for 10 minutes. Cool. Store in an airtight container.