01 - Set oven temperature to 350°F (175°C).
02 - Combine graham cracker crumbs and sugar in a medium bowl; stir in melted butter until mixture is evenly moistened. Press mixture firmly into bottom and up sides of a 9-inch pie dish. Bake for 8 to 10 minutes until lightly golden. Allow to cool slightly.
03 - In a medium saucepan, whisk together sugar, cornstarch, and salt. Gradually add water while whisking until smooth. Cook over medium heat, stirring constantly, until mixture thickens and reaches a boil, about 5 minutes.
04 - Whisk egg yolks in a separate bowl. Slowly add 1/2 cup hot lemon mixture to yolks while whisking to temper. Return yolk mixture to saucepan, continue whisking and boil for 1 additional minute. Remove from heat.
05 - Stir in fresh lemon juice, grated lemon zest, and unsalted butter until filling is smooth and glossy. Pour filling into prepared crust and smooth surface.
06 - Beat egg whites and cream of tartar in a clean bowl using an electric mixer at medium-high speed until soft peaks form. Gradually add sugar, beating until stiff, glossy peaks develop. Mix in vanilla extract.
07 - Spread meringue evenly over hot lemon filling, ensuring meringue seals to crust edges to prevent shrinking. Use the back of a spoon to create decorative swirls.
08 - Bake assembled pie at 350°F (175°C) for 15 to 18 minutes, or until meringue is golden brown.
09 - Allow pie to cool to room temperature, then refrigerate for at least 2 hours before serving to achieve clean slices.