01 - Preheat oven to 350°F. Position the rack in the center of the oven.
02 - In a mixing bowl, combine the graham cracker crumbs, 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 the sugar, cornstarch, and salt until well blended. Pour in the water, freshly squeezed lemon juice, and lemon zest. Place over medium heat and whisk constantly until the mixture thickens and begins to bubble, approximately 6 to 8 minutes.
04 - In a small bowl, lightly beat the egg yolks. While whisking continuously, slowly drizzle in about 2 tablespoons of the hot lemon mixture to gently warm the yolks without scrambling them. Pour the tempered yolk mixture back into the saucepan, stirring constantly. Continue cooking for 2 more minutes until the filling is thick and glossy.
05 - Remove the saucepan from the heat. Add the softened butter and stir until fully melted and the filling is smooth and silky. Pour the warm lemon filling into the cooled crust, spreading it evenly with a spatula.
06 - In a clean, dry 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 sugar one tablespoon at a time. Increase the speed to high and beat until stiff, glossy peaks hold their shape.
07 - Spread the meringue over the warm lemon filling, making sure to seal the edges against the crust to prevent shrinking. Create decorative peaks and swirls using the back of a spoon. Bake for 15 minutes until the meringue topping is lightly golden.
08 - 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 to allow the filling to set properly.