Mix the cornstarch, salt, and ½cup sugar in the top of a double boiler. Gradually add the water, stirring. Place over boiling water and cook, stirring constantly, until the mixture thickens. Cover and cook 10 minutes, stirring occasionally. Do not remove from the heat.
Beat the egg yolks in a mixing bowl and add ½cup sugar. Stir a small amount of the hot mixture into the egg yolks. Immediately pour the egg yolk mixture into the remaining hot mixture over boiling water; blend thoroughly. Cook 2 minutes longer, stirring constantly. Remove from the heat and add the butter, lemon rind, and lemon juice. Cool to room temperature without stirring.
Turn the filling into the baked pie shell. Beat the egg whites until stiff but not dry. Gradually beat in the remaining 6tablespoons sugar. Spread the meringue over the filling, being sure to extend it over the end of the crust.
Bake until delicately browned, 15 to 20 minutes. Cool to room temperature and serve.