Heat the oven to 350°F (175°C). Combine the graham cracker crumbs, sugar, melted butter, and optional salt, then press the mixture firmly into the bottom and slightly up the sides of a 9-inch pie plate.

Bake the crust for 8–10 minutes, until it is lightly browned and fragrant. Set it aside to cool while preparing the lemon filling.

Whisk the sugar, cornstarch, and salt together in a saucepan. Gradually whisk in the water until the mixture is smooth and no dry pockets remain.

Cook the mixture over medium heat, whisking frequently, until it thickens and reaches a full boil. Continue boiling for about 1 minute.

Whisk the egg yolks in a bowl. Slowly whisk about 1 cup of the hot sugar mixture into the yolks to temper them, then return the tempered mixture to the saucepan.

Cook the filling for 2–3 minutes, whisking constantly, until thick and glossy. Remove the saucepan from the heat and stir in the lemon juice, lemon zest, and butter.

Pour the hot lemon filling into the baked graham cracker crust. Keep the filling hot while preparing the meringue so the topping can adhere properly.

Beat the egg whites and cream of tartar until foamy. Gradually add the sugar and continue beating until glossy, stiff peaks form, then beat in the vanilla if using.

Spread the meringue over the hot lemon filling, taking it all the way to the edge of the crust. Press and seal the meringue firmly against the crust to help prevent shrinking.

Bake the pie at 350°F for 12–15 minutes, until the meringue is golden brown. Cool at room temperature for about 1 hour, then refrigerate for at least 2 hours before slicing.