In a small bowl mix cold water and cornstarch to a thin paste. In a medium saucepan bring hot water and sugar to a boil, then add cornstarch paste.
Transfer this mixture to a double boiler. Cook until thick and smooth, stirring constantly, about 15 minutes.
Whisk a small amount of this mixture into egg yolks, then whisk egg yolk mixture back into double boiler. Cook everything together for a few minutes longer, continuing to stir.
Mix in juice and rind of 1 lemon along with butter. Set custard aside to cool.
In a medium glass or metal mixing bowl, beat egg whites while gradually adding sugar, 1 tablespoon at a time. When egg whites hold a stiff peak, beat 1 teaspoon lemon juice into meringue.
Pour filling into baked pie shell.
Spread meringue over lemon filling.
Bake at 325 degrees F (165 degrees C) for 25 to 30 minutes or until delicately browned.
