Combine sugar and corn starch in a medium-size heavy saucepan; mix well. Add water; bring to a boil. Reduce heat; cook, stirring constantly, until slightly thickened.
Beat yolks until thick and lemon colored. Gradually stir one-fourth of hot mixture into yolks; add to remaining hot mixture. Continue cooking over medium heat, stirring constantly, until sauce thickens.
Remove from heat. Add butter and lemon juice; stir until well blended. Chill until ready to use.