Prepare the lemon mixture. In a medium saucepan, whisk together 1 cup of granulated sugar, 2 tablespoons of flour, 3 tablespoons of cornstarch, and ¼ teaspoon of salt. Stir in 1 ½ cups of water, the juice, and zest of 2 lemons. Cook over medium-high heat, stirring frequently, until the mixture comes to a boil.
Thicken the filling. After the mixture boils, turn down the heat and let it simmer for 1 minute. Keep stirring until the mixture thickens a lot.
Temper the egg yolks. In a small bowl, whisk 4 beaten egg yolks. Gradually whisk in about ½ cup of the hot sugar mixture to temper the egg yolks. This step prevents the eggs from curdling.
Combine and cook. Pour the tempered egg yolks back into the saucepan. Bring the mixture to a gentle boil and continue cooking, stirring constantly, for 2 minutes. The filling should be thick and smooth.
Finish the filling. Remove from heat. Add 2 tablespoons of butter and keep stirring until it melts completely and mixes into the filling.
Cool and use. Let the filling cool a bit before pouring it into your pie crust. If you're not using it right away, cover the filling with plastic wrap pressed onto the surface to avoid forming a skin.