01 - Preheat oven to 350°F. Lightly grease a 9-inch pie pan and set aside.
02 - In a mixing bowl, combine graham cracker crumbs, sugar, and melted butter until the mixture resembles wet sand. Press firmly and evenly into the bottom and up the sides of the pie pan.
03 - Bake the crust for 8 minutes until set and lightly golden. Remove from the oven and let cool while preparing the filling.
04 - In a medium saucepan, whisk together sugar, cornstarch, salt, and lemon zest until evenly combined.
05 - Gradually whisk in the lemon juice, egg yolks, and water into the dry mixture, stirring until completely smooth with no lumps.
06 - 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 immediately.
07 - Add butter to the hot filling and whisk vigorously until fully melted and incorporated, creating a smooth, glossy curd.
08 - Pour the hot lemon filling directly into the prepared graham cracker crust, spreading evenly with a spatula.
09 - In a clean, grease-free mixing bowl, beat egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form. Gradually add sugar, one tablespoon at a time, beating on high speed until stiff, glossy peaks hold their shape.
10 - Spread the meringue over the hot filling, making sure to seal the edges completely against the crust to prevent shrinking. Create decorative swirls or peaks using the back of a spoon.
11 - Bake for 12 to 15 minutes until the meringue is golden brown on the peaks and edges. Watch carefully to avoid over-browning.
12 - Cool the pie at room temperature for 1 hour, then refrigerate for at least 2 hours before slicing to allow the filling to set completely.