Make the crust: In a bowl, whisk flour, sugar, and salt. Cut in the cold butter until pea-sized bits remain.
Drizzle in ice water, tossing just until the dough holds together. Form a disk, wrap, and chill 30–45 minutes.
Par-bake the crust: Heat oven to 400°F (200°C). Roll dough to a 12-inch circle and fit into a 9-inch pie dish.
Prick the base with a fork. Line with parchment and fill with pie weights. Bake 15 minutes, remove weights, and bake 8–10 more minutes until lightly golden.
Cool slightly. Reduce oven to 350°F (175°C).
Prep the meringue slurry: In a small saucepan, whisk cornstarch and water until smooth. Cook over medium heat, stirring, until thick and translucent, 1–2 minutes.
Remove from heat and keep warm; this stabilizer helps prevent weeping.
Start the lemon filling: In a medium saucepan, whisk sugar, cornstarch, and salt. Gradually whisk in water until smooth. Cook over medium heat, stirring constantly, until thick and bubbling.
Let it bubble for 30 seconds.
Temper the yolks: In a bowl, whisk egg yolks. Slowly whisk in about 1/2 cup of the hot mixture. Return yolk mixture to the pan, whisking.
Cook 1–2 minutes, stirring, until glossy and very thick.
Finish the filling: Remove from heat. Stir in lemon juice, zest, and butter until smooth. Pour the hot filling directly into the warm pie crust.
Keep it hot; meringue adheres best to hot filling.
Make the meringue: In a clean bowl, beat egg whites and cream of tartar on medium speed until soft peaks. Gradually add sugar, 1 tablespoon at a time, beating to stiff, glossy peaks. Beat in vanilla.
Beat in the warm cornstarch slurry on low until fully incorporated.
Top the pie: Spoon meringue over the hot lemon filling, spreading to the very edges to seal the crust. Create swoops with a spatula for nice peaks.
Bake: Bake at 350°F (175°C) for 15–18 minutes, until the meringue is lightly golden on top. Don’t over-brown.
Cool on a rack for 1 hour, then chill 3–4 hours to set.
Serve: Slice with a sharp, clean knife. Wipe the blade between cuts for neat slices.