Mix sugar, cornstarch and salt in a saucepan. Stir in water slowly stirring with a whisk until smooth. Cook, stirring constantly, over low heat until thickened.
Add a small amount of hot mixture to beaten egg yolks to temper. Then add yolks to custard mixture.
Cook for approximately 1 more minute.
Remove from heat and add lemon rind, juice and butter. Stir until blended.
Pour into baked crust, top with meringue and baked until lightly browned.