In large bowl, whisk flour with salt.

Using a fork, mix in grated butter until pieces of butter are coated in flour.

In glass measure, whisk egg yolk with lemon juice; add enough ice water to make 1/3 cup. Whisk to combine; drizzle over flour mixture, tossing with fork and adding up to 1 tsp more cold water, if necessary, until ragged dough forms.

Shape dough into disc; wrap and refrigerate until chilled, about 30 minutes. (Make ahead: Refrigerate for up to 3 days.)

On lightly floured surface, roll out pastry to 1/8-inch (3 mm) thickness. Fit into 9-inch (23 cm) pie plate. If necessary, trim edge to leave 1-inch (2.5 cm) overhang. Line pastry with foil; fill with pie weights or dried beans.

Bake on bottom rack in 400 F oven for 15 minutes. Remove foil and pie weights; using fork, prick pastry all over. Bake until golden, 10 to 12 minutes. Let cool in pan on rack.

In heavy-bottomed saucepan, whisk together sugar, cornstarch and salt; whisk in water. Bring to boil over medium-high heat, whisking constantly.

Reduce heat to medium-low; simmer, whisking constantly, for 3 minutes. Remove from heat. Gradually whisk one-quarter of the sugar mixture into egg yolks; whisk back into pan. Cook over medium heat, whisking constantly, for 2 minutes. Remove from heat; whisk in lemon zest, lemon juice, vanilla, and butter until butter is melted. Let cool for 30 minutes. Scrape into pie shell and smooth top.

In bowl, beat egg whites with salt until soft peaks form.

Beat in sugar, 1 tbsp at a time, until stiff peaks form.

Starting at edge and using spatula, spread some of the meringue around outer edge of filling, sealing meringue to crust.

Using a spatula, spread meringue over remaining filling, making peaks.

Bake in 400 F (200 C) oven until meringue is golden, 5 to 6 minutes. Let cool in pan on rack until completely set, about 5 hours.