Preheat the oven to 400°F. Place a baking sheet on the middle rack. Pierce the bottom of the pie crust with a fork. Cover the base with foil and then place 1/4 cup pie weights or dried beans inside. Freeze 5 minutes before baking.
Place pie dish on baking sheet in oven. Bake 20 minutes. Remove foil and beans and brush edges with an egg wash (see notes) if desired. Cook an additional 5 to 10 minutes, or until crust is golden. Reduce the oven temperature to 325℉.
Lemon Filling
In a medium bowl, whisk the egg yolks until foamy.
In a medium saucepan, whisk together cornstarch, sugar, salt, and water. Bring to a boil over medium heat, whisking constantly. Simmer 1 to 2 minutes, or until well thickened. Remove from heat.
Whisk 1 teaspoon cornstarch mixture into egg yolks until combined. Repeat until 1/2 of the cornstarch mixture is whisked into the eggs. Pour the egg mixture back into the saucepan and whisk to combine.
Whisking continuously, bring to a boil over medium heat 3 to 4 minutes. Remove from heat and stir in lemon zest, lemon juice, and butter until combined and smooth. Cover to keep warm while making the meringue.
Meringue
Using a handheld mixer or a stand mixer fitted with a whisk attachment, beat together the egg whites and cream of tartar on medium speed 1 minute. Increase to high speed and beat 4 minutes, or until soft peaks form
Add the sugar, salt, and vanilla and continue beating on high speed 2 minutes more, or until glossy stiff peaks form.
Assemble + Bake
Spread the warm lemon filling evenly into the pie crust. If the filling is no longer warm, reheat it while whisking constantly. The warm filling makes it easier for the meringue to adhere to the top.
Spread meringue evenly over the top of the filling (making decorative swoops, if you like), being sure the meringue touches the edge of the crust to help prevent weeping.
Bake on the lowest rack 20 to 25 minutes, or until golden. If the meringue starts to brown too quickly, tent with foil but being careful not to touch the meringue. Let cool 1 hour on a wire rack before transferring to the fridge to chill 4 hours.
Notes
Baking tips:
Eggs: Separate the eggs while they are cold. Use the egg yolks in the filling and leave the egg whites out while you blind bake the pie crust. By the time you make the meringue, the egg whites will be room temperature.
Warm filling: It's important to spread the meringue topping on while the filling is still warm. It will seal the two layers together, preventing separation.
Meringue prep: Make sure there is not even a drip of egg yolks in the egg whites. The bowl must be completely clean and dry, with No oil or water residue.
Humidity: The weather will make it more difficult to make meringue, so I would recommend making a different dessert on a humid day.
Storage: This lemon meringue pie tastes is best eaten the day it is made. You can cover and store the leftovers in the fridge, but the meringue may wilt and separate.