In 2-quart saucepan, mix 1 cup sugar, 2 tablespoons cornstarch, the flour and 1/4 teaspoon salt. Gradually stir in 1 1/2 cups 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 juice. Cook over medium heat about 5 minutes, stirring constantly. Remove from heat. Add butter; stir until melted. Let stand 10 minutes.