Zest 2 to 3 lemons for at least 2 tablespoons of zest. Juice lemons to obtain 3/4 cup of juice
Whisk together the 4 egg yolks in a medium bowl and set aside
Add sugar, cornstarch, and salt to a medium pot. Whisk to combine then pour in water, lemon juice and zest and whisk again to distribute all the dry ingredients. You don't want clumps
Place on medium heat and whisk continuously until thickened. About 5 to 10 minutes.
Time to temper the eggs! slowly drizzle in a cup of the hot lemon mixture to the eggs while whisking continuously. Add the egg mixture to the pot while whisking and return to medium-high heat for 2 minutes of whisking over heat.
Add in butter and stir until all is melted. Pour into cooked and cooled pie crust.
At this point, you'll want to let the pie cool and even refrigerate until the lemon curd or custard is set.
for the Meringue
Combine sugar, cornstarch and water in a small saucepan over medium heat. Continue stirring continuously until dissolved and thickened. Set aside while...
Beat the egg whites and salt with an electric or stand mixer fitted with the whisk attachment and slowly add 4 tablespoons of sugar and continue beating until soft peaks form.
Drizzle the cornstarch mixture and the vanilla into the meringue and turn mixer to high speed, beating until firm peaks form. Use immediately.
Add meringue onto pie leaving a small border of the lemon surd exposed. use your spatula or a spoon to create some funky designs and peaks for the visual as we eat with our eyes first.
You can toast the meringue with a kitchen torch or broil for about 1 to 2 minutes or until meringue is golden. Chill until pie is set before cutting, about 3 hours.