1 (9 inch) deep pie shell, baked
1 ½ cups cold water
1 cup sugar
¼ cup cornstarch
⅛ tsp salt
6 large egg yolks
1 tbsp grated fresh lemon zest
½ cup fresh lemon juice (3 lemons)
2 tbsps unsalted butter, cut into 2 pieces
¾ cup sugar
⅓ cup water
3 large egg whites, room temperature
¼ tsp cream of tartar
¼ tsp vanilla
For the filling: Bring the water, sugar, cornstarch, and salt to a simmer in a large saucepan over medium heat, whisking constantly. When the mixture starts to turn translucent, whisk in the egg yolks, 2 at a time. Whisk in the lemon zest, then the lemon juice, and finally the butter. Return the mixture to a brief simmer, then remove the pan from the heat.
Pour filling into cooled, baked pie crust. Lay a sheet of plastic wrap directly on the surface of the filling and refrigerate the pie until the filling is cold, about two hours.
For the meringue: Adjust an oven rack to the middle position and heat the oven to 400ºF. Bring the sugar and water to a boil over medium high heat and cook until the mixture is slightly thickened and syrupy (about 235ºF on a candy thermometer), 3 to 4 minutes. Remove from the heat and cover to keep warm.
In a large bowl, whip the egg whites, cream of tartar, and salt together with an electric mixer on medium low speed until foamy, about 1 minute. Increase the mixer speed to medium high and whip the whites until shiny and soft peaks form, 1 to 3 minutes.
Reduce mixer speed to medium and slowly add the hot syrup, avoiding the whisk and the sides of the bowl. Add the vanilla, increase the mixer speed to medium high, and continue to whip the mixture until it has cooled slightly and is very thick and shiny, 3 to 6 minutes.
Dollop the meringue evenly over the top of the chilled pie filling* and spread it into a tidy looking mound, making sure it adheres to the crust. Use the back of a spoon to create attractive swirls and peaks in the meringue. Bake until the meringue is golden brown, about 6 minutes. Let the pie cool on a wire rack until the filling has set, about 2 hours. Serve at room temperature.
*Sometimes you place the meringue on a warm pie filling if the recipe calls for it.