Zest 2 lemons, making sure the zest is fine and the lemon juice about 1/4 cup.
In a medium saucepan stir together the corn starch, flour, 1 1/2 cups of sugar and 1/8 tsp of salt with a wooden spoon. Pour in water and stir constantly over medium low heat until mixture comes to a slow boil. Be sure not to burn mixture on the bottom of the pan.
Remove from heat. Place 5 egg yolks in a small bowl and add 1 cup of the heated corn starch mixture and whisk well. Place the egg yolk mixture back into the sauce pan with the heated corn starch mixture and return to low heat and cook for another 3 minutes- stirring constantly with your wooden spoon. Remove from heat and stir in the lemon juice and lemon zest. Lastly, whisk in the sliced butter, one piece at a time. Pour filling into baked and cooled shell.
Whip together 5 egg whites, vanilla and a pinch of salt until soft peaks form. Slowly add 1 1/4 cups of sugar while you continue to beat. Dollup meringue on top of pie. Gently spread, making sure to touch all sides of the pie crust with the meringue.
Bake at 325 for 20-25 minutes, watching closely until the meringue is lightly browned. Cool pie and place in refrigerator before serving and for storing afterwards.
Here are some tips for a fuller Meringue, note, do not use copper bowls.