Combine the sugar, flour, cornstarch, and salt and mix together.
Gradually add the water, stir and cook the ingredients over low heat until the mixture gently boils. It should thicken and become smooth.
Beat the egg yolks together in a bowl, then slowly and constantly whisking, add in 1 cup of the hot sugar mixture. Once incorporated, slowly add the warmed egg mixture back to the saucepan containing the remaining sugar mixture, whisking the entire time. Cook over low heat for three minutes, constantly stirring.
Stir in the butter, lemon juice, and lemon rind.
Allow to cool on stovetop, then pour custard into the baked pie shell and cool in the refrigerator.
Preheat oven to 425 degrees (F).
It's easiest to use an electric mixer to make the meringue. Beat the egg whites with the salt in a small bowl until light and frothy.
Add the cream of tartar and continue to beat at high speed until the mixture will stand in peaks, about 4 - 5 minutes.
Add the vanilla extract and beat in the sugar, 1 tablespoon at a time.
Cover the chilled pie filling with the meringue until it touches the edges of the pastry to prevent the meringue from shrinking.
Bake at 425 degrees (F) for five minutes. Cool and then refrigerate until cold before serving.
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