01 - Preheat oven to 350°F. Position the rack in the center position.
02 - In a mixing bowl, combine the graham cracker crumbs, sugar, and melted butter, stirring 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.
03 - Bake the crust for 8 to 10 minutes until lightly golden and set. Remove from the oven and set aside to cool completely.
04 - In a medium saucepan, whisk together the sugar, cornstarch, and salt until evenly blended. Pour in the water, lemon juice, and lemon zest, then whisk until smooth.
05 - Place the saucepan over medium heat and cook, whisking constantly, until the mixture thickens and begins to bubble, approximately 6 to 8 minutes.
06 - In a small bowl, whisk the egg yolks lightly. Gradually whisk in a spoonful of the hot lemon mixture to temper the yolks, then pour the warmed yolk mixture back into the saucepan while stirring continuously. Cook for 2 more minutes, stirring constantly, until thickened and glossy.
07 - Remove the saucepan from the heat and stir in the softened butter until fully melted and the filling is silky smooth. Pour the lemon filling into the cooled crust, spreading it into an even layer.
08 - In a clean, grease-free bowl, beat the egg whites and cream of tartar with an electric mixer until soft peaks form. Gradually add the sugar, about one tablespoon at a time, beating continuously until stiff, glossy peaks hold their shape.
09 - Spread the meringue over the warm filling, making sure to seal the edges against the crust to prevent shrinking. Bake for 15 minutes until the meringue peaks are golden brown.
10 - Allow the pie to cool at room temperature for 1 hour, then transfer to the refrigerator and chill for at least 1 hour before slicing and serving.