1TO MAKE FILLING:
Mix sugar, salt, cornstarch. Add water and lemon juice. Cook over medium heat stirring with a whisk until thick and clear.
2Place egg yolks in a small bowl and very gradually whisk in 1/2 cup of hot sugar mixture. Go slowly so yolks do not cook. Then slowly whisk egg yolk mixture back into remaining sugar mixture. Bring to a boil and continue to cook while stirring constantly until thick. Remove from heat. Add butter and cool a bit. Pour into pie shell.
3TO MAKE MERINGUE:
With mixer beat egg whites until frothy; gradually add sugar, continuing to beat until stiff peaks form. Add vanilla. Spoon on top of lemon pie filling, making peaks if you like and spreading to crust edge to seal filling in. Bake at 325 degrees for 15 to 18 minutes, until meringue is nicely browned.
4NOTE: Depending on the color you like for the filling you may add a bit of yellow food coloring along with the vanilla.