Butter sides of a heavy 2-quart saucepan. In pan combine sugar, milk, half-and-half, and corn syrup. Cook and stir over medium-high heat till mixture boils. Clip a candy thermometer to pan. Reduce heat to medium-low; continue boiling at a moderate, steady rate, stirring frequently, till thermometer registers 238 degrees F, soft-ball stage (25 to 35 minutes).