Step 1

Preheat the oven to 425°F. Line the shaped, chilled, pie crust with parchment or foil and fill with weights (I like reusing dried beans for this purpose) and bake for 15 minutes on the lower rack. Remove the liner and weights and bake until the whole crust is golden brown. Remove from the oven and allow to cool.

Step 2

Place the lemon juice, water, and zest in a medium sized saucepan. Whisk together the cornstarch, 9 ounces (1 1/3 cups) sugar, and salt, and add it to the saucepan. Whisk the egg yolks then add them to the mixture and whisk continuously over medium heat until the mixture becomes thick and bubbles, 7 to 8 minutes. Whisk for an additional two minutes once it's begun to bubble, without stopping, then remove from heat. Whisk in the butter until it is completely melted. Pour the mixture into the baked pie shell and press plastic wrap directly onto the surface. Allow the pie to come to room temperature, then chill for at least four hours (ideally, overnight) with the plastic wrap still on top.

Step 3

Before serving: Fill medium saucepan one quarter full with water. Set the saucepan over medium heat, and bring water to a simmer.Combine egg whites, remaining sugar, and cream of tartar in the heatproof bowl of electric mixer, and place over saucepan. Whisk constantly until sugar is dissolved and whites are warm to the touch, 3 to 3 1/2 minutes. Test by rubbing between your fingers. Transfer bowl to electric mixer fitted with the whisk attachment, and whip, starting on low speed, gradually increasing to high until stiff, glossy peaks form, about 10 minutes. Add vanilla, and mix until combined. Remove the plastic from the top of the chilled pie and top with the meringue. Place the pie under the broiler and watch if carefully, the top should brown but not burn, and it changes quickly. Chill for 10 minutes, then serve.