01 - Preheat oven to 350°F (175°C).
02 - In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter. Stir until the mixture resembles wet sand. Press the crumb mixture firmly and evenly into the bottom and up the sides of a 9-inch pie pan.
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 granulated sugar, cornstarch, salt, and lemon zest until evenly combined. Gradually whisk in the freshly squeezed lemon juice, egg yolks, and water, stirring until the mixture is completely smooth.
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 unsalted 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 granulated sugar, about 1 tablespoon at a time, and continue beating on high speed until stiff, glossy peaks hold their shape.
08 - Spread the meringue evenly over the hot lemon filling, making sure to seal the edges against the crust to prevent shrinking. 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 to allow the filling to set properly.