In a saucepan, stir together the sugar, cornstarch, salt and water. Cook on medium heat until the mixture comes to a boil and bubbles, stirring often. Cook for 3 more minutes, stirring continuously. Turn the heat off.
Whisk the yolks in a separate bowl. Whisk in 1/2 cup of the hot mixture bit by bit, then whisk the yolk mixture into the saucepan. Let the filling return to a gentle boil and cook for 3 more minutes, stirring continuously. Turn off the heat. Add the lemon zest, lemon juice and the butter. Stir until it is smooth and let cool slightly.
Pour the cooled lemon mixture into a baked pie or tart shell and chill until set.
For the Meringue:
Whip 2 egg whites and add sugar to taste part way through. Whites should be at room temperature to whip. Whip until firm peaks form when you pull out a spoon.
Spoon over lemon pie. Bake at 350 degrees until the top is browned. Watch carefully so it doesn't burn.