Butter sides of a heavy 2-quart saucepan. In saucepan melt butter. Add sugar, water, and corn syrup. Cook and stir over medium-high heat until mixture boils. Clip a candy thermometer to side of pan. Reduce heat to medium; continue boiling at a moderate, steady rate, stirring frequently, till thermometer registers 290 degree F, soft-crack stage (about 15 minutes). Watch carefully after 280 degree F to prevent scorching. Remove saucepan from heat; remove thermometer. Pour candy into the prepared pan.