Ensure your pie crust is fully baked and cooled to room temperature.

In a medium saucepan, whisk together 1 cup sugar, cornstarch, and salt. Stir in the water and cook over medium heat, stirring occasionally until thickened. Once it starts to boil, remove it from the heat.

In a separate bowl, beat the egg yolks lightly. Gradually add about 1/2 cup of the hot mixture to temper the yolks, stirring continuously. Then pour the tempered yolks back into the saucepan. Cook for an additional two minutes, stirring constantly.

Remove the saucepan from heat and stir in the lemon juice, lemon zest, and butter until fully blended.

Pour the lemon filling into the pre-baked pie crust, spreading it evenly.

In a clean bowl, beat the egg whites until frothy.

Add cream of tartar and continue beating until soft peaks form.

Gradually add 1/2 cup sugar, beating until stiff peaks form.

Spread the meringue over the lemon filling, sealing to the edges to prevent shrinking.

Bake in a preheated oven at 350°F (175°C) for about 10-12 minutes, or until the meringue is golden brown.

Allow the pie to cool at room temperature before slicing and serving.