In a boiler, combine sugar and corn starch well. (A whisk is best!)
Slowly add water and continue stirring until mixture comes to a boil and begins to thicken.
Temper egg yolks and add to mixture.
Bring to a boil and cook for 1 – 2 minutes.
Stir in lemon zest and juice.
Remove from heat and stir in butter one tablespoon at a time.
Pour filling into pie crust.