Mix cornstarch, sugar and salt in saucepan; gradually stir in hot water. Cook over direct heat, stirring constantly, until thick and clear, about 10 minutes.
Remove from heat. Stir ½ c. hot mixture into yolks; stir this back into hot mixture. Cook over low heat; stirring constantly, 2 to 3 minutes. Remove from heat; stir in butter. Add lemon peel and juice, stirr