01 - Heat oven to 375°F.
02 - Combine flour, salt, and sugar with cold butter until mixture resembles coarse crumbs. Add egg yolk and ice water; mix just until dough forms. Shape into a disk, wrap, and chill 30 minutes.
03 - Roll dough to fit a 9-inch pie dish. Trim edges, prick base, line with parchment, and fill with baking beans. Bake 15 minutes, remove beans and parchment, then bake 10 minutes more until lightly golden. Cool completely.
04 - Whisk together sugar, cornstarch, and salt in a saucepan. Gradually add water and cook over medium heat, stirring until thick and bubbling, then cook an additional minute.
05 - Whisk egg yolks separately. Slowly whisk a few spoonfuls of the hot mixture into yolks, then return all to the saucepan. Cook for 2 minutes, stirring constantly.
06 - Remove from heat and stir in lemon juice, zest, and butter until smooth. Pour filling into cooled crust.
07 - Beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, beating to stiff glossy peaks. Fold in vanilla extract.
08 - Spread meringue over hot lemon filling, sealing edges to crust and creating decorative peaks.
09 - Bake 10–12 minutes until meringue is golden. Cool to room temperature, then chill at least 2 hours before serving.