Preheat your oven to 350°F (175°C). In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter until the mixture resembles wet sand. Press this mixture into a prepared pie pan and bake for about 10 minutes until lightly golden. Allow to cool slightly.

In a large bowl, whisk together sweetened condensed milk and egg yolks until smooth. Slowly add lemon juice and optional lemon zest. Pour the filling into the crust and bake for 15–18 minutes until edges are set. Cool at room temperature, then refrigerate for at least 4 hours.

Set up a double boiler. Whisk egg whites, granulated sugar, cream of tartar, and salt until it reaches 140°F on a candy thermometer. Remove from heat and whip with a hand mixer until stiff peaks form, gently folding in vanilla extract.

Dollop the meringue over the chilled lemon filling, covering the entire surface and creating peaks and swirls. Be generous with the meringue.

Use a kitchen torch to lightly toast the meringue until golden brown. Alternatively, place under a low broiler for a few minutes, watching closely to prevent burning.

Let the pie cool slightly. Slice and serve it chilled. Optionally add whipped cream or lemon zest on each slice.