Combine sugar, cornstarch, and salt in a saucepan.

Add water to the mixture and stir well.

Bring the mixture to a boil over medium heat, stirring constantly until it thickens and turns a translucent color.

Remove from heat.

In a separate bowl, lightly beat the egg yolks.

Slowly whisk a small amount of the hot sugar mixture into the egg yolks to temper them.

Pour the tempered egg yolk mixture back into the saucepan with the remaining sugar mixture.

Return the saucepan to low heat and cook for 6 minutes, stirring constantly.

Remove from heat.

Add the butter, grated lemon rind, and lemon juice to the mixture.

Stir until the butter is melted and all ingredients are well combined.

Allow the lemon filling to cool slightly.

Pour the cooled lemon filling into a pre-baked pie shell.

Refrigerate the pie for at least 1 hour before serving to allow the filling to set completely.