In a medium saucepan, whisk together sugar, cornstarch, and water. Cook over medium heat, stirring constantly, until mixture thickens and begins to boil.
Stir in lemon juice and butter. Gradually whisk in beaten egg yolks. Return to heat and cook for 2 more minutes, stirring constantly.
Pour the mixture into the pre-baked pie crust.
Bake in the preheated oven for 10 minutes. Allow to cool before serving.