01 - Preheat oven to 350°F. Position the rack in the center position.
02 - In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter. Stir until the mixture resembles wet sand and holds together when pressed. Transfer to a 9-inch pie pan and press firmly and evenly across the bottom and up the sides. Bake for 8 to 10 minutes until lightly golden. Remove from the oven and set aside to cool completely.
03 - In a medium saucepan, whisk together granulated sugar, cornstarch, and salt until evenly blended. Pour in the water, freshly squeezed lemon juice, and lemon zest. Set the saucepan over medium heat and whisk constantly for 6 to 8 minutes, until the mixture thickens and begins to bubble.
04 - In a small bowl, lightly whisk the egg yolks. Slowly drizzle in about ¼ cup of the hot lemon mixture while whisking vigorously to temper the yolks without scrambling them. Pour the warmed yolk mixture back into the saucepan, stirring continuously. Cook for an additional 2 minutes, stirring constantly, until the filling is glossy and thick.
05 - Remove the saucepan from heat. Add the softened butter and stir until fully melted and the filling is silky smooth. Pour the hot lemon filling directly into the cooled graham cracker crust, spreading it into an even layer with a spatula.
06 - 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. With the mixer running, gradually add the granulated sugar, about 1 tablespoon at a time. Increase to high speed and beat until stiff, glossy peaks hold their shape.
07 - Spoon the meringue over the hot lemon filling, spreading it to seal completely against the crust edges to prevent shrinkage. Use the back of a spoon to create decorative swirls. Bake for 15 minutes until the meringue is lightly golden on the peaks. If skipping the meringue, the pie is ready to cool as-is.
08 - Allow the pie to cool at room temperature for 1 hour. Transfer to the refrigerator and chill for at least 1 additional hour, or until the filling is fully set. Slice into 8 wedges and serve chilled, garnished with lemon slices or zest if desired.