01 - Combine graham cracker crumbs, sugar, melted butter, and salt in a bowl. Press mixture firmly into the bottom and up the sides of a 9-inch pie dish.
02 - Bake crust at 350°F for 8 to 10 minutes until set and lightly golden. Remove and cool.
03 - Whisk sugar, cornstarch, and salt in a saucepan. Gradually add water, lemon juice, and zest, stirring to combine.
04 - Cook over medium heat, stirring constantly until mixture thickens and boils. Boil for 1 minute more.
05 - Beat egg yolks in a small bowl. Slowly whisk in about 1/2 cup of hot lemon mixture to temper, then return all to the pan.
06 - Cook, stirring, for 2 minutes more. Remove from heat and stir in butter until melted. Pour filling into cooled crust.
07 - Beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually add sugar one tablespoon at a time, beating to stiff, glossy peaks. Beat in vanilla.
08 - Spread meringue evenly over hot lemon filling, sealing edges to crust. Bake at 350°F for 15 to 18 minutes until meringue is golden.
09 - Cool at room temperature for 1 hour. Refrigerate at least 2 hours before serving.