Pie crust:
In a medium bowl, stir together the flour, sugar, and salt.
Cut in butter with a pastry cutter or fork until you see pea-sized chunks of butter remaining.
Gradually add in cold water, stirring each time until you can press it together.
Form it into a disk, wrap in plastic wrap, and refrigerate for at least 2 hours, or freeze for 30 minutes until chilled.
Preheat oven to 425°F (220°C).
Roll out to fit a 9″ pie plate. Press into the pie plate and over the sides. Trim the edges and crimp or leave as desired.
Poke the bottom and the sides with a fork all over to help prevent puffing (or use pie weights if you have them).
Bake for 10-12 minutes until light golden brown. Set aside while you make the filling.
Filling:
In a medium saucepan, whisk together the sugar, water, corn starch, egg yolks, lemon juice, and lemon zest.
Cook over medium – medium-high heat, whisking constantly, until thickened — it will be thick, slightly thicker than pudding when warm. This can take 10-15 minutes, but don’t rush it.
Remove from heat and stir in butter.
Pour hot filling into the pie crust and make the meringue.
Meringue:
Preheat oven to 375°F (190°C).
In a small saucepan, whisk together the water and corn starch. Cook over medium heat until thickened and clear. Set aside but keep warm.
In a clean bowl with clean beaters, beat egg whites, vanilla, and salt until soft peaks form.
Gradually add in the sugar, beating until stiff glossy peaks form.
Gradually add in the warm corn starch mixture, beating constantly, until completely incorporated and meringue is light and fluffy and holds stiff peaks.
Spread meringue onto hot pie filling (it’s important that the filling is still hot!) and spread right to the edge of the pie crust. The meringue must touch the edges all the way around.
Bake at 375°F just until the tops of the meringue are light golden brown — don’t overbake.
Let rest on the counter on a wire rack for 1-2 hours, then refrigerate for 5-6 hours until chilled, or overnight.
Slice and serve.