Whisk the sugar, cornstarch, salt and water in a large (nonreactive) saucepan. Heat over medium heat until the mixture thickens and reaches a simmer, stirring fairly often. When the mixture starts to simmer and turns translucent, whisk in the egg yolks, one at a time. Then whisk in the zest, then the lemon juice, and then the butter. Bring the mixture to a good simmer, whisking constantly. Remove from the heat.

Place a sheet of plastic wrap directly on the surface of the filling to keep it hot and prevent a skin from forming.

In a small saucepan, whisk together the cornstarch and water until combined. Bring the mixture to a simmer over medium-low heat, whisking occasionally at the beginning and more frequently as the mixture thickens. Once the mixture starts to simmer and turns translucent, remove from heat and set aside.

Adjust an oven rack to the middle position and heat the oven to 325 degrees.

In a small bowl, whisk the cream of tartar and sugar together. In the bowl of a large stand mixer, beat the egg whites and vanilla for 1-2 minutes on medium-high speed until frothy. Beat in the sugar mixture, 1 tablespoon at a time, until the sugar is completely incorporated. Continue beating on medium speed until the mixture forms soft peaks. (*Look ahead to multitask the next step with the lemon filling at this point.) Add the cornstarch mixture, 1 tablespoon at a time, while the mixture is still beating. Continue to beat on medium speed until the mixture forms stiff peaks, about 1-2 minutes.

*When you are about a minute away from finishing the meringue, remove the plastic from the lemon filling and return the saucepan to medium heat. Let it heat for at least 1 minute so that it is nice and hot when you add it to the pie shell.

Once your meringue is ready and the filling is hot, pour the filling into the prebaked pie shell and use a rubber spatula to even out the top. Use a spoon to immediately add small spoonfuls of the meringue all around the edge of the filling, and then add spoonfuls to the center until the entire surface of the filling is covered. Make sure the meringue attaches to the piecrust to prevent shrinking. Use the back of a spoon to create little decorative peaks all over the meringue, or whatever design you would like.

Bake the pie until the meringue is lightly browned, about 20 minutes. Transfer to a wire rack and cool to room temperature. Serve the same day.

*Be sure that the eggs you use to make the meringue are fresh. Otherwise, they may not whip up properly.