Prepare the Crust:
Preheat the oven to 375°F (190°C).
In a large mixing bowl, combine flour and salt. Cut the cold butter into small cubes and mix it into the flour until the mixture resembles coarse crumbs.
Gradually add cold water, one tablespoon at a time, and mix until the dough comes together.
Roll the dough out on a floured surface and fit it into a 9-inch pie dish. Trim and crimp the edges. Prick the bottom of the crust with a fork.
Bake the crust in the preheated oven for 15-20 minutes or until golden. Set aside to cool.
Prepare the Lemon Filling:
In a saucepan, whisk together the sugar, cornstarch, and water. Bring to a boil over medium heat, stirring constantly until the mixture thickens.
In a separate bowl, whisk the egg yolks. Gradually pour some of the hot mixture into the egg yolks to temper them, then slowly whisk the tempered eggs back into the saucepan.
Continue to cook over medium heat until the mixture thickens further. Remove from heat, then stir in lemon juice, lemon zest, and butter until smooth.
Pour the lemon filling into the pre-baked pie crust.
Prepare the Meringue:
In a clean mixing bowl, beat the egg whites and cream of tartar with an electric mixer until soft peaks form.
Gradually add sugar, one tablespoon at a time, until stiff peaks form and the meringue is glossy.
Spoon the meringue over the lemon filling, spreading it all the way to the edges of the crust to seal it.
Bake the Pie:
Bake the pie in the oven at 350°F (175°C) for 10-15 minutes, or until the meringue is golden brown.
Remove from the oven and let it cool for at least 2 hours before serving.