Heat oven to 375°F. In 2-quart saucepan, mix 2/3 cup sugar and the cornstarch. Gradually stir in cold water until blended. In small bowl, beat egg yolks slightly with fork. Stir yolks into sugar mixture in saucepan. Cook over medium heat 10 minutes, stirring constantly, until mixture boils. Boil 1 minute, stirring constantly. Remove from heat. Stir in lemon peel, lemon juice and butter until well blended.