For the Crust:

1. Preheat the oven to 375°F and set an oven rack in the middle position.
2. In a medium bowl, mix together the graham cracker crumbs and sugar. Add the melted butter; stir with a fork first, and then your hands until the mixture is well combined. Using your fingers and the bottom of a clean drinking glass or measuring cup, press the crumbs firmly into the bottom and up the sides of a 9 x 1.5-inch pie pan. The crust should be about ¼ inch thick.
3. Bake for about 10 minutes, until just slightly browned. Let the crust sit on a wire rack while you prepared the rest of the pie. Lower the oven temperature to 325°F.

For the Filling:

1. In a medium bowl, whisk together the egg yolks, sweetened condensed milk, lemon zest, and lemon juice. Pour into the warm crust.

For the Meringue:

1. Bring about ½ inch of water to a simmer in a pot that's large enough to hold the bowl of a stand mixer without letting it touch the water. Reduce the heat to low. Put the egg whites and sugar in the bowl off the heat and whisk until frothy. Put the bowl over the pot and whisk gently but constantly until the whites are very warm to the touch, or about 115°F (the temperature does not need to be exact; the mixture should feel hot, but not so hot that you can't leave your finger in there) and the sugar is completely dissolved, 2 to 4 minutes.
2. Remove the bowl from the heat and transfer it to the stand mixer fit with the whisk attachment. Add the cream of tartar. Beat over medium high speed until the egg whites form thick, glossy medium-firm peaks (they should hold their shape but curl at their tips), 3 to 5 minutes total.
3. Mound the meringue onto the wet lemon filling. Using a soup spoon, gently spread the meringue over the entire surface of the filling, making sure to go all the way to the crust’s edge (it is essential that the edge of the meringue be anchored to the rim of the crust; otherwise it will shrink). Make decorative peaks and swirls in the meringue with the back of the spoon.
4. (Be sure you've reduced the oven temperature to 325°F.) Bake for about 20 minutes, or until the meringue is lightly browned. Let the pie cool completely on a rack before serving, about 3 hours.
5. Lemon meringue pie is best eaten the day it’s made, but leftovers will keep, loosely tented with aluminum foil and refrigerated, for 2 days. Just keep in mind that meringue gets quite sticky in the fridge. Do not freeze.
6. Note: Several readers have had issues with the filling being too runny. I believe this is likely caused by using less common brands of sweetened condensed milk, such as Meadow Gold or Weis. Please use Eagle brand sweetened condensed milk for reliable results. Also, for Canadians, please note that Canadian Eagle sweetened condensed milk comes in a smaller-sized can than the American version (you will need two cans but you won't use all of it).