Bake the Crust: Preheat oven to 375°F. Prick the bottom of a frozen pie crust with the tines of a fork. Cover with a piece of parchment paper. Fill with pie weights or beans. Bake in the preheated oven for 10 minutes then remove the parchment paper. Continue to bake for another 10-15 minutes or until golden brown. Set aside to cool on a wire rack.
Filling: In a medium saucepan whisk together sugar, cornstarch and salt. Add water and lemon juice.
Cook the hot sugar mixture whisking gently over medium-high heat until sugar is dissolved and mixture has thickened.
Meanwhile, in a small bowl whisk egg yolks. Temper egg yolks by drizzling 1/4 cup warm liquid into beaten yolks while constantly whisking.
Lower to medium heat. Whisking constantly gradually add tempered egg yolks to saucepan.
Bring the mixture to a simmer and cook for 2-3 minutes while stirring with a spatula. The mixture should be thick. Remove from heat adding butter and zest. Mix until completely melted and fully blended.
Pour the filling evenly into the crust. Cover with plastic wrap while you prepare the meringue.
Meringue: Using an electric mixer using the whisk attachment, beat egg whites with cream of tarter and vanilla on medium-high speed until foamy. Slowly add sugar while beating. Beat until all sugar has been added and stiff peaks form, this will take several minutes. (Tip: The slower you add the sugar it will dissolve more easily and prevent a gritty texture.)
Remove wrap plastic from pie. Spoon and spread the meringue on top of the lemon filling taking it completely to the crust.
Transfer to the oven and bake the meringue at 350°F for 8-10 minutes until lightly golden brown, or broil for several minutes, rotating the pan for even browning. Don't walk away.
Chill for at least 4-6 hours before slicing.