01 - Preheat oven to 350°F.
02 - Combine graham cracker crumbs, sugar, and melted butter in a medium bowl until evenly moistened.
03 - Press mixture firmly into bottom and up sides of a 9-inch pie plate.
04 - Bake for 8 minutes. Remove from oven and cool while preparing filling.
05 - Whisk sugar, cornstarch, and salt in a medium saucepan. Gradually whisk in water.
06 - Cook over medium heat, stirring constantly, until mixture thickens and boils gently, 5–7 minutes.
07 - Remove from heat. Slowly whisk 1/2 cup hot mixture into beaten yolks, then return yolk mixture to saucepan.
08 - Return to medium heat and cook, whisking constantly, for 2 minutes. Remove from heat and stir in lemon juice, zest, and butter until smooth.
09 - Pour hot filling into cooled crust. Set aside.
10 - Beat egg whites and cream of tartar at high speed until soft peaks form.
11 - Gradually add sugar, 1 tablespoon at a time, beating until stiff, glossy peaks form. Beat in vanilla.
12 - Spread meringue over hot filling, sealing edges carefully to crust to prevent shrinking.
13 - Bake for 15–18 minutes until meringue is golden brown.
14 - Cool at room temperature for 1 hour, then refrigerate at least 2 hours before slicing.