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1
For the filling: In a medium saucepan combine cornstarch, water, lemon juice, sugar, and salt. Whisk until no lumps remain in the cornstarch and bring the mixture to a boil, stirring frequently. Continue to cook, stirring constantly, for 75 seconds. The mixture will be clear and very thick.
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2
Whisk the egg yolks together in a medium bowl and slowly whisk the cornstarch mixture into the egg yolks while stirring constantly. Return the mixture to the saucepan and cook over low heat while stirring constantly for 60 more seconds. Be careful not to let the mixture boil at this stage.
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3
Remove the pan from the heat and whisk in the butter and lemon zest. Pour the filling into the prepared pie shell and cover the filling with a piece of parchment or waxed paper. This will help the meringue and filling adhere later. Set it in the refrigerator while you prepare the Swiss meringue.
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4
For the meringue: Whisk the egg whites and sugar together in the bowl of a stand mixer. Place the bowl of the stand mixer over a double boiler and cook, stirring occasionally, until the egg whites are very warm to the touch and the sugar has dissolved.
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5
Preheat your broiler. While the broiler is heating, whip the egg whites on medium-high until stiff peaks form. Add in the salt and vanilla extract and whip to combine well.
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6
Remove the parchment paper from the pie filling then carefully spread the meringue on top of the lemon filling in a decorative pattern. Make sure to spread the meringue all of the way to the crust.
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7
Carefully slide the pie under the broiler and cook for 20 to 60 seconds, or until the meringue is golden brown with some deeply caramelized spots. Remove the pie from the oven and cool completely in the refrigerator before slicing and serving. This pie is best served the day that it is made.
