01 - Preheat oven to 350°F. Set aside a 9-inch pie pan.
02 - In a mixing bowl, combine graham cracker crumbs, sugar, and melted butter until evenly moistened. Press the mixture firmly into the bottom and along the sides of the pie pan to form an even crust.
03 - Bake the crust for 8 minutes. Remove from the oven and set aside to cool while preparing the filling.
04 - In a medium saucepan, whisk together sugar, cornstarch, salt, and lemon zest until combined. Gradually whisk in the lemon juice, egg yolks, and water, stirring until the mixture is smooth and free of lumps.
05 - Place the saucepan over medium heat and whisk constantly for 6 to 8 minutes, until the mixture thickens significantly and comes to a gentle boil. Remove from heat and whisk in the butter until fully melted and incorporated.
06 - Pour the hot lemon filling directly into the prepared graham cracker crust, spreading it into an even layer.
07 - In a clean, grease-free mixing bowl, beat the egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form. Gradually add the sugar, about 1 tablespoon at a time, and continue beating until stiff, glossy peaks hold their shape.
08 - Spread the meringue evenly over the hot filling, making sure to seal the edges against the crust. Bake for 12 to 15 minutes, or until the meringue peaks are golden brown.
09 - Allow the pie to cool completely at room temperature, then refrigerate for at least 2 hours before slicing and serving.