In a saucepan, mix 1/2 cup sugar, cornstarch, and salt. Gradually add lemon juice and water, stirring until smooth.
Cook over medium heat until the mixture is thickened and bubbly, then reduce the heat and cook for 2 more minutes.
Gradually whisk in the egg yolks, stirring constantly. Cook and stir for another 2 minutes, then remove from heat.
Stir in butter and lemon zest. Transfer to a bowl, cover with plastic wrap directly on the surface, and refrigerate until cool, about 1 hour.
Beat egg whites with a mixer until soft peaks form. Gradually add the remaining 1/2 cup sugar, beating until stiff peaks form.
Fold the lemon mixture into the beaten egg whites until no streaks remain.
Spoon the filling into the baked pie shell. Chill for at least 3 hours or until set.
Serve chilled. Optionally, garnish with whipped cream or lemon slices.