In a saucepan, combine sugar, cornstarch.
Add cold water, mix. Stir in hot water.
Cook over medium heat stirring constantly until mixture is thick and clear.
Allow to bubble for 1 minute.
Beat egg yolks. Stir a small amount of hot mixture in with the egg yolks, add more gradually and then return all of the warmed mixture to the remaining portion in the saucepan (this prevents eggs from being scrambled).
Bring to a boil and cook for 1 minute more, stirring constantly.
Remove from the heat, add butter and lemon rind.
Slowly stir in lemon juice. Cool to lukewarm before pouring into pastry shell.
Top with meringue.