Prepare the Filling: In a medium-sized mixing bowl, whisk four egg yolks and set aside.
Cook the Mixture: In a medium saucepan, combine cornstarch, water, sugar, and salt. Stir over medium-low heat until the mixture boils, then allow to boil for one minute. Subsequently, reduce the heat to low.
Temper the Egg Yolks: Gradually pour half of the hot mixture into the whisked egg yolks, ensuring constant whisking to avoid curdling. Then, return this combined mixture to the saucepan, continuing to whisk.
Finalize the Filling: Return the saucepan to the stove, cooking for an additional three minutes while stirring constantly. Remove from heat, then incorporate the lemon juice, lemon zest, and butter, one tablespoon at a time. Stir occasionally until the mixture cools.
Prepare for Baking: Preheat the oven to 350 degrees Fahrenheit. Grease and flour a mini-muffin pan, or use nonstick cooking spray.
