In the work bowl of a food processor, pulse flour and salt until combined. Add cold butter, and pulse until mixture is crumbly and butter is pea-size. With processor running, add ⅓ cup (80 grams) ice water in a slow, steady stream just until dough comes together. (Mixture may appear crumbly. It should be moist and hold together when pinched. Add more ice water if needed.)

Turn out dough, and press into a flat disk. Wrap tightly in plastic wrap. Refrigerate for at least 1 hour or up to 2 days. Beyond 2 days, dough can be frozen for up to 3 months. If using dough from frozen, let thaw in refrigerator overnight.

Preheat oven to 425°F (220°C).

Top dough with a piece of parchment paper, letting ends extend over edges of plate. Add pie weights.

Bake until edges are golden brown, 15 to 17 minutes. Remove parchment and weights, and bake until bottom of crust is dry and golden, 5 to 7 minutes more. Let cool completely.

In a medium saucepan, whisk together sugar and cornstarch. Add egg yolks and egg, and whisk vigorously until combined. Whisk in lemon zest and juice, ⅓ cup (80 grams) water, and salt until completely combined and no clumps remain. Heat over medium-low heat, whisking constantly, until mixture begins to bubble and thicken, about 7 minutes; cook for 2 minutes. Remove from heat, and strain through a fine-mesh sieve into a heatproof bowl. Add butter, and stir until butter is completely incorporated.

Pour filling into cooled prepared crust. Cover with plastic wrap, pressing wrap directly onto surface of filling to prevent a skin from forming. Refrigerate until completely cooled, at least 1 hour.

Remove plastic wrap from cooled filling; top with Swiss Meringue as desired. Using a handheld kitchen torch, carefully brown meringue as desired. Best served same day.

In the top of a double boiler, whisk together egg whites, sugar, salt, and cream of tartar. Cook over simmering water, stirring occasionally, until sugar dissolves and an instant-read thermometer registers 160°F (170°C), about 7 minutes.

Transfer mixture to the bowl of a stand mixer. Using the whisk attachment, beat at high speed until tripled in volume and stiff peaks form, 3 to 4 minutes. Beat in vanilla. Use immediately.