01 - Preheat oven to 350°F. Position rack in the center position.
02 - Combine graham cracker crumbs, sugar, and melted butter in a mixing bowl, 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. Bake for 8-10 minutes until lightly golden. Remove and set aside to cool completely.
03 - In a medium saucepan, whisk together sugar, cornstarch, and salt until no lumps remain. Pour in water, freshly squeezed lemon juice, and lemon zest. Place over medium heat and whisk constantly for 6-8 minutes until the mixture thickens and begins to bubble.
04 - In a small bowl, whisk the egg yolks until smooth. Slowly drizzle about 1/4 cup of the hot lemon mixture into the yolks while whisking vigorously to temper them. Pour the warmed 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 heat. Stir in the softened butter until fully melted and the filling is silky smooth. Pour the hot lemon filling directly into the cooled crust, spreading evenly with a spatula.
06 - In a spotlessly clean bowl, beat egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form. With the mixer running, gradually add sugar one tablespoon at a time. Increase speed to high and beat until stiff, glossy peaks hold their shape. Spread the meringue over the hot filling, making sure to seal it completely to the crust edges to prevent shrinking.
07 - Bake for 15 minutes until the meringue topping is lightly golden with toasted peaks. If skipping the meringue, tent the exposed filling loosely with foil during the last few minutes to prevent over-browning.
08 - Allow the pie to cool at room temperature for 1 hour. Transfer to the refrigerator and chill for at least 1 hour or until the filling is fully set before slicing and serving.