To make the filling, combine milk and sugar in a nonreactive saucepan, preferably enameled iron. Strip the zest from the lemons with a sharp vegetable peeler, making sure you remove the yellow zest but none of the white pith beneath. If you do remove some of the white pith, scrape it off the strips of zest with the point of a paring knife and discard it. Add the zest to the milk and sugar and bring to a simmer over low heat. Remove from heat and allow to steep for 5 minutes; remove the strips of zest with a slotted spoon or skimmer and discard them.
Squeeze lemons to make 1/2 cup strained juice. Place juice in a mixing bowl and whisk in cornstarch, then yolks. Return milk and sugar mixture to a boil over low heat and whisk about a third of the boiling milk into the lemon juice mixture.
Return remaining milk and sugar mixture to a boil once more and whisk the lemon juice and yolk mixture back into it, whisking constantly until the filling comes to a boil and thickens. Allow to boil, whisking constantly, for about 30 seconds. Remove from heat, whisk in butter and pour into a nonreactive bowl. Press plastic wrap against the surface of the filling and chill until it is approximately 75 degrees. (If you prepare the filling in advance, let it come to room temperature before proceeding.)
Set a rack at the middle level of the oven and preheat to 350 degrees. Roll out the dough to make bottom crust and arrange in pan. Chill crust until firm.
To bake the crust, pierce it all over with the tines of a fork at 1/2-inch intervals. Line it with a disk of parchment or waxed paper and fill with cherry stones or dried beans. Bake about 20 minutes, until lightly colored. Remove paper and beans and continue baking until the crust is a deep golden brown. Cool crust on a rack.
Spread the cooled filling evenly in the cooled crust.
Set a rack at the middle level of the oven and preheat to 400 degrees.
To make the meringue bring a small pan of water to a boil. Lower heat so that water simmers. Combine egg whites, sugar and salt in the bowl of the mixer or if you are using a hand whisk another heatproof bowl. Place bowl over pan of simmering water and whisk gently for about 2 minutes, until egg whites are hot (about 140 degrees) and sugar has dis solved. Whip meringue on medium speed until it has cooled and is able to hold a shape, but it should not be dry. Distribute spoonfuls of the meringue all over the top of the pie, then use the back of a spoon or a small offset metal spatula to spread the meringue evenly. It should cover the top of the pie and touch the edges of the crust all around. Here and there, bring up the surface of the meringue so that it is swirled. Place the pie on a cookie sheet and bake for about 5 to 10 minutes, until the meringue is colored evenly. Cool on a rack.
To mix the dough by hand, combine flour, salt and baking powder in a medium-sized mixing bowl and stir well to mix. Cut butter into 1tablespoon pieces and add to dry ingredients. Toss once or twice to coat pieces of butter. Then using your hands or a pastry blender, break the butter into tiny pieces and pinch and squeeze it into the dry ingredients. Keep the mixture uniform by occasionally reaching down to the bottom of the bowl and mixing all the ingredients evenly together. Continue rubbing the butter into the dry ingredients until the mixture resembles a coarse-ground cornmeal and no large pieces of butter remain visible.
Sprinkle the minimum amount of water over the butter and flour mixture and stir gently with a fork the dough should begin holding together. If the mixture still appears dry and crumbly, add the remaining water, 1 teaspoon at a time for the smaller quantity of dough, a tablespoon at a time for the larger quantity, until the dough holds together easily.
To mix the dough in the food processor, combine flour, salt and baking powder in work bowl fitted with metal blade. Pulse 3 times at 1second intervals to mix. Cut butter into 1 tablespoon pieces and add to work bowl. Process, pulsing repeatedly at 1second intervals, until the mixture is fine and powdery, resembles a coarse-ground cornmeal and no large pieces of butter remain visible about 15 pulses in all.
Scatter the minimum amount of water on the butter and flour mixture and pulse 5 or 6 times the dough should begin holding together. If the mixture still appears dry and crumbly, add the remaining water, 1 teaspoon at a time for the smaller quantity of dough,one tablespoon at a time for the larger quantity, until the dough holds together easily.
Turn the dough out onto a lightly floured surface and form it into a disk (two equal disks for the larger amount of dough). Sandwich the disk(s) of dough between two pieces of plastic wrap and press it into a 6inch circle. Refrigerate the dough until firm, or until you are ready to use it, at least 1 hour.