In a medium saucepan, combine sugar, cornstarch, and water. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil for 1 minute.
Remove from heat. Stir in lemon juice, butter, and lemon zest. Gradually stir in beaten egg yolks. Return to heat and cook for 2 more minutes, stirring constantly. Pour into pre-baked pie crust.
Bake in preheated oven for 10 minutes, or until filling is set. Let cool before serving.