Bake the pie crust:

1. Preheat oven to 425F.
2. If using homemade pie dough, roll it out to be about 12 inches in diameter. Carefully transfer it into a 9-inch pie dish or a 9-inch tart pan and finish by trimming the edges.
3. Prick the assembled pie dough with a fork all over. Place a crumpled piece of parchment inside, then fill it with pie weights or dried beans.
4. Place onto a large baking sheet and then into the oven to bake for 18 minutes. After 18 minutes, remove the pie crust from the oven. Carefully remove the parchment paper and weights.
5. Turn the oven down to 375°F, and then return the crust to the oven to bake for 12 more minutes. Remove the crust from the oven, brush with egg wash (if using), then return to the oven for 2 more minutes. Finally, remove from the oven, and allow the crust to cool completely.

Make the lemon filling:

1. In a medium mixing bowl, add the sugar and lemon zest and use your fingers to rub them together until very fragrant.
2. Transfer the zested sugar to a medium saucepan along with the egg yolks, water, lemon juice, cornstarch, and salt. Whisk until very smooth, then place on the stove over medium-low heat. While whisking constantly, bring the lemon pie filling up to a simmer, then continue to cook and whisk until thickened (like pudding) and about 170-175°F in temperature as read on a food thermometer (do not exceed 180°F, or it will curdle).
3. Immediately remove from the heat, and stir in the cold butter cubes.
4. Place a fine mesh sieve over top of the cooled pie crust, then pour the pie filling into the sieve, using a spatula to coax the filling through the sieve.
5. Place a piece of plastic cling wrap over the surface of the pie, then transfer to the fridge to fully cool – at least 4 hours, but preferably overnight.

Make the meringue:

1. When ready to serve, add egg whites, white sugar, and cream of tartar, to the bowl of your stand mixer, and whisk together. Place the bowl over top of a simmering pot of water (bain-marie), ensuring a tight seal and ensuring that the bottom of the bowl is not actually touching the water.
2. Heat the mixture over the bain-marie until it reaches 165°F, continuously whisking while it heats up. When temperature is reached, remove the bowl from the water and transfer to your stand mixer attached with the whip attachment.
3. Beat the meringue on high until stiff peaks have formed and the meringue is no longer warm. Add vanilla, then beat for 10 more seconds to incorporate.
4. Dollop the meringue on top of the lemon pie until it's covered completely.
5. Use a kitchen torch and torch the meringue until golden. If you do not have a kitchen torch, you can place the pie under your oven broiler for about 30 seconds, or until browned (be sure to keep an eye on it though, as things can burn quickly).
6. Let the lemon meringue pie cool completely at room temperature before refrigerating.