Makes 1 9-inch pie. Filling and meringue adapted from The Art of the Pie by Kate McDermott.
Preheat oven to 350 degrees. Grease a 9-inch round tart pan with removable bottom and fluted edges.
In a bowl, stir together the butter, powdered sugar, flour, and salt until it makes a soft, crumbly dough. Pour the dough into the prepared tart pan, and using your fingers, press the dough firmly into the bottom of the pan and up the sides. Prick the crust several times with a fork. Bake for 15-20 minutes, until set. Set the crust aside to cool.
*Note: At this stage, be sure to separate your eggs for the filling and the meringue and juice and zest your lemons before proceeding. Have your other ingredients out and in place because you will need to have everything accessible as you work through these next steps.
In a saucepan, stir together the sugar, cornstarch and salt. Whisk in the water and turn the heat onto medium-high. Bring the mixture to a boil, whisking constantly. Reduce the heat to medium-low and continue whisking as it thickens, about 2 minutes. Remove the pan briefly from the heat. In a small bowl, take 1/4 cup of the hot thickened mixture and whisk it quickly into the beaten egg yolks to temper the eggs. Then, return the saucepan to the heat and quickly whisk in the egg yolk mixture until yellow and smooth. Whisk in the butter, lemon juice, and zest, cooking for another minute until the butter is melted and the filling is smooth and fragrant. Pour the filling directly into the prepared tart crust. Now, promptly make the meringue.
Using a stand mixer fitted with the whisk attachment, beat the egg whites on high speed. Gradually add the salt, sugar, and cream of tartar while the egg whites are beating. Beat until the meringue is bright white and fluffy, holding soft peaks. Using an offset spatula, spread the meringue onto the warm filling, making swoops and peaks. Place the pie into the oven and bake for 6 minutes or until the peaks of the meringue begin to brown and the meringue is just set.
Remove the pie from the oven and allow it to cool completely before slicing and serving.