Blind bake a pie crust.
In a medium mixing bowl whisk the egg yolks together and set aside.
Whisk the water, sugar, cornstarch, salt, lemon juice and lemon zest together in a heavy, medium size saucepan over medium heat.
The mixture will be thin and cloudy, then eventually begin thickening and bubbling after about 6 minutes.
Once thickened, give it a whisk and reduce heat to low.
Very slowly stream a few large spoonful's of warm lemon mixture into the beaten egg yolks, while continually whisking.
Then very slowly pour the mixture into the sauce pan, whisking while pouring it into the pot.
Turn the heat back up to medium cook and whisk until mixture is thick and big bubbles begin bursting to the surface.
Remove the pan from the heat and whisk in the butter until melted.
Spread the filling into the warm, prepared pie crust.
Set aside while you make the meringue.
Don't let the filling cool down too much, you want a warm filling when you top with the meringue.
The warm filling helps seal the 2 layers together, preventing separation.
Chef's notes: Prepare all of your ingredients before you begin, including grating the lemon zest and separating the eggs. The filling is time sensitive and you want to make sure everything is ready when you need to add it. Prep all the meringue ingredients as well. You want them on hand, especially the sugar and salt. See recipe for meringue below.