In 2-quart saucepan, mix sugar, cornstarch and flour. Gradually stir in water, stirring until smooth. Heat to boiling over medium heat; cook and stir 1 minute longer. Remove from heat. Quickly stir about 1/2 cup hot mixture into beaten egg yolks; mix well. Gradually stir egg mixture back into hot mixture. Stir in lemon peel and lemon juice. Cook over medium heat about 5 minutes, stirring constantly. Remove from heat. Add butter; stir until melted. Let stand 10 minutes.