In a medium saucepan, combine sugar and cornstarch; gradually whisk in water. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil and whisk for 1 minute. Slowly whisk half the hot mixture into the beaten egg yolks. Blend egg mixture back into saucepan. Continue to heat to a boil, stirring constantly, for 1 minute. Remove from heat; whisk in butter, lemon zest, lemon juice and salt.