Make sure all of the ingredients are as cold as possible. In the bowl of the food processor, mix the flour, sugar and salt. Add all of the pieces of butter and pulse about 10 to 15 times, until the mixture looks like coarse crumbs. Drizzle in the ice water and continue processing until the mixture begins to stick together.
Pour the mixture out into a bowl or a floured surface and press together into a disk. Wrap in plastic and chill for at least 20 minutes.
Bring the dough to room temperature if it is too hard to roll. On a lightly floured surface, roll to about ⅛-inch thick. Cut a circle in the dough so that it is 2 inches larger than the pie pan, then transfer the dough to the pan. Turn the edges under, leaving an edge that hangs over about ½ inch. Flute the edges and chill for 30 minutes.
Set the oven rack in the middle and preheat to 350° F. Line the crust with foil and fill with dried beans or pie weights. Bake for 20 to 25 minutes. Remove the foil and weights and continue baking for another 10 to 15 minutes until the crust is golden. Remove from the oven and cool completely on a wire rack before filling.
In the heavy sauce pan, bring the water to a boil. Remove from heat and let it rest for 5 minutes. Whisk together the sugar and cornstarch. Gradually the mixture to the hot water, whisking until the dry ingredients have dissolved.
Return the pan to the cooktop and cook over medium heat, whisking constantly until the mixture starts to boil. The mixture will be very thick.
In the medium bowl, beat the egg yolks. Add 1 cup of the hot mixture to the egg yolks and whisk until smooth. Add the yolks to the hot pot, whisking vigorously, and continue cooking. Stir constantly until the mixture comes to a boil. Remove from heat and add the butter, stirring until melted and incorporated. Add the lemon juice, zest, and vanilla, and stir until combined. Pour into the prepared crust. Cover with plastic wrap to prevent a skin from forming on the surface and cool to room temperature.
Meringue & Bake
Preheat the oven to 375° F.
In the bowl of the stand mixer, beat the egg white, cream of tartar, salt, and vanilla with the wire whisk until soft peaks form. Slowly add the sugar while beating, until you have stiff, glossy peaks. Place large dollops of the meringue around the top of the filled pie, bringing the meringue to the edges to seal in the filling. Tap the top of the meringue lightly all around with a spoon, creating curls and peaks.
Bake for 15 to 20 minutes, until the meringue is golden brown. Cool on a rack, then store in the refrigerator. Best eaten the same day.