1. Make the crust:

In a bowl, whisk together flour, sugar, and salt. Cut in butter with a pastry cutter or your fingers until the mixture resembles coarse crumbs. Add ice water 1 tbsp at a time, mixing until the dough comes together. Form into a disc, wrap, and chill for 30 minutes. Roll out and press into a 9-inch pie dish, trim the edges, and chill again for 15 minutes. Preheat oven to 375°F, line crust with parchment and pie weights, and blind bake for 20 minutes. Remove weights and bake another 10 minutes until golden. Cool completely.

2. Make the lemon filling:

In a saucepan, whisk sugar, cornstarch, and salt. Gradually whisk in water. Cook over medium heat, stirring constantly, until it begins to thicken and bubble. In a separate bowl, whisk the egg yolks. Slowly add a bit of the hot mixture to the yolks (to temper), then return it all to the saucepan. Cook for 1–2 more minutes until thick. Remove from heat and stir in lemon zest, juice, and butter. Pour into the cooled crust.

3. Make the meringue:

Using a mixer, beat egg whites and cream of tartar on medium-high until soft peaks form. Gradually add sugar, 1 tbsp at a time, and continue beating until glossy stiff peaks form. Beat in vanilla if using.

4. Assemble and bake:

Spread the meringue over the hot lemon filling, sealing the edges to the crust to prevent shrinking. Use a spatula to create swirls. Bake at 350°F for 15–20 minutes until the meringue is golden. Cool completely at room temperature, then chill for at least 2 hours before slicing.