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Reduce the oven's heat to 350F (175C). Whisk together the sugar, flour, cornstarch, and salt in a medium-sized saucepan, then slowly add the water. Cook the mixture over medium-high heat until it comes to a boil. Beat the egg yolks together in a bowl, then add cup (120 milliliters) of the hot mixture to the yolks. Add the egg yolk mixture to the saucepan, return it to a boil, and cook until it thickens. Remove the saucepan from heat and add the butter, lemon juice, and lemon zest. Pour the mixture into the pie shell and set it aside.

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Keep whisking until the flour and cornstarch are dissolved. Don't turn on the stove just yet. Keep whisking it through so that it doesn't scorch. Add the mixture slowly, beating it in with a whisk. Save the egg whites to make the meringue topping. Continue whisking the mixture. It will take about 1 minute for it to thicken. Keep stirring until butter is melted. For extra flavor, add teaspoon of vanilla extract. Don't store it in the fridge; you want the filling to be still warm when you spoon the meringue on top.