01 - Preheat the oven 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 firmly into the bottom and up the sides of a 9-inch pie dish. Bake for 8 to 10 minutes until lightly golden. Allow to cool slightly.
03 - Whisk together sugar, cornstarch, and salt in a medium saucepan. Gradually add water, stirring until smooth. Cook over medium heat, whisking constantly until the mixture thickens and boils, about 5 minutes.
04 - In a separate bowl, whisk egg yolks. Slowly whisk in 1/2 cup of the hot lemon mixture to temper the yolks. Return the yolk mixture to the saucepan, whisking constantly. Bring to a boil and cook for 1 minute, then remove from heat.
05 - Stir in fresh lemon juice, lemon zest, and unsalted butter until smooth and glossy. Pour the filling into the prepared crust and level the surface.
06 - Beat egg whites and cream of tartar on medium-high speed until soft peaks form. Gradually add sugar, continuing to beat until stiff, glossy peaks form. Fold in vanilla extract.
07 - Spread the meringue over the hot lemon filling, ensuring it seals to the crust edges to prevent shrinking. Create decorative swirls using the back of a spoon.
08 - Bake the pie at 350°F (175°C) for 15 to 18 minutes, or until the meringue is golden brown.
09 - Allow the pie to cool to room temperature, then refrigerate for at least 2 hours to set before serving.