Combine sugar and cornstarch; gradually stir in water. Cook over medium heat, stirring constantly, until mixture thickens and boils. Continue boiling 1 minute
Gradually stir one-fourth of hot mixture into egg yolks; add to remaining hot mixture, stirring constantly. Return to a boil; cook 1 to 2 minutes longer, stirring constantly. Remove from heat and continue stirring until smooth. Stir in butter, lemon rind, and lemon juice. Cool.