Prepare Pie Crust: Preheat oven to 375°F (190°C). Combine flour and salt in a bowl. Cut in cold butter until mixture resembles coarse crumbs. Gradually add ice water until dough forms. Roll out and fit into a 9-inch pie pan. Prick bottom with a fork. Bake 10–12 minutes until lightly golden. Cool completely.

Make Lemon Filling: In a medium saucepan, whisk together sugar, flour, cornstarch, and salt. Stir in water, lemon juice, and zest. Cook over medium heat, stirring frequently, until mixture comes to a boil. Boil for 1 minute. Remove from heat. Gradually whisk in butter and egg yolks. Return to heat and cook 2 minutes, stirring constantly, until thickened. Pour filling into cooled pie crust.

Prepare Meringue: In a clean bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, 1 tbsp at a time, beating until stiff, glossy peaks form. Beat in vanilla.

Top Pie: Spread meringue over lemon filling, sealing edges to crust to prevent shrinking.

Bake: Reduce oven to 350°F (175°C) and bake 10–15 minutes, until meringue is golden brown.

Cool: Cool to room temperature, then refrigerate for at least 2 hours before serving to allow filling to set.