Preheat your oven to 350°F (175°C) to ensure even baking.

In a food processor, pulse together the graham cracker crumbs, granulated sugar, and melted butter until the mixture resembles wet sand. Press this mixture firmly into the bottom of a lined pie pan. Bake for 10 minutes, then let it cool.

In a mixing bowl, whisk together the sweetened condensed milk and egg yolks until smooth. Gradually stir in the lemon juice and optional lemon zest, then pour the mixture into your baked graham cracker crust. Bake for 15-18 minutes until the edges are set.

Let the pie cool at room temperature for 30 minutes, then transfer it to the refrigerator to chill for at least 4 hours.

Set up a double boiler. Combine the egg whites, granulated sugar, cream of tartar, and salt in a mixing bowl. Whisk until warm, then whip until stiff peaks form. Add vanilla at the end.

Spread the meringue over the chilled lemon filling, making peaks with a spatula or spoon.

Optionally, use a kitchen torch to lightly toast the meringue until golden brown, or broil in the oven for a few minutes, watching closely to avoid burning.

Slice the pie and serve chilled for the best flavor.