Preheat oven to 350°F.

Filling

1. Beat the egg yolks in a spacious bowl until smooth. Set aside.
2. Combine the cornstarch, sugar and salt together in a medium saucepan.
3. Add the water and lemon juice and whisk until smooth.
4. Whisk in the egg yolks, butter and lemon zest.
5. Set over a medium heat and whisk constantly until the mixture boils and thickens, approximately 10 minutes. Adjust heat to prevent scorching.
6. Whisk in a drop or two of yellow food coloring if using.NOTE: Whisk well to eliminate any lumps in the filling.
7. Pour the filling into the prepared crust. Set aside.

Meringue

1. Place the egg whites and cream of tartar in the clean bowl of an electric mixer.
   NOTE: In order to achieve maximum volume, egg whites should be brought to room temperature before whipping. Since grease interferes with the frothing action, make sure that your bowl and utensils are perfectly clean and avoid using plastic bowls, which can harbor traces of grease even after thorough cleaning. If in doubt, rub the bowl and whisk attachment with a half lemon. Similarly, even a hint of stray yolk mixed in the whites will prevent proper whipping, so use care when separating the eggs.
2. Beat together until soft peaks form.
3. Slowly add the sugar and beat until stiff, shiny peaks form.
4. Scoop the meringue on top of the lemon filling, forming a mound over the pie.
5. Place on middle oven rack and bake until the meringue is golden, approximately 20 minutes.
6. Cool the pie to room temperature, then refrigerate 4–6 hours or overnight.
7. Remove from the refrigerator and serve.