In a medium saucepan, combine 1 cup of granulated sugar, 1/4 cup of cornstarch, and 1/4 teaspoon of salt.
Gradually stir in 1 1/2 cups of water until the mixture is smooth.
Cook over medium heat, stirring frequently, until the mixture comes to a boil.
Boil for 1 minute, then remove from heat.
Gradually whisk about half of the hot mixture into 4 beaten egg yolks, then return the yolk mixture to the saucepan, stirring constantly.
Bring back to a gentle boil and cook for an additional minute, or until the mixture has thickened.
Remove from heat and stir in 3/4 cup of fresh lemon juice, 1 tablespoon of grated lemon zest, and 2 tablespoons of unsalted butter until well combined. Let the lemon filling cool until it’s just warm.
Prepare the Meringue:
Preheat your oven to 325°F (163°C).
In a large, clean mixing bowl, beat 4 egg whites and 1/4 teaspoon of cream of tartar until soft peaks form.
Gradually add 1/2 cup of granulated sugar and continue beating until stiff peaks form.
Combine and Bake:
Gently fold the beaten egg whites into the cooled lemon filling to lighten it.
Pour the mixture into a baked 9-inch pie crust, spreading it evenly.
Bake for 25 to 30 minutes, or until the center is set and the top is lightly golden.
Cool and Chill:
Remove the pie from the oven and let it cool on a wire rack.
Once cooled, chill the pie in the refrigerator for at least 4 hours before serving.
