In a medium-size heavy bottomed saucepan whisk together sugar, cornstarch and salt. Add water and lemon juice.
Cook whisking gently over medium-high heat until sugar is dissolved and mixture has thickened.
Meanwhile, in a separate bowl, whisk egg yolks. Temper egg yolks by drizzling ¼ cup warm liquid into beaten yolks while constantly whisking.
Lower heat to medium. Whisking constantly gradually add tempered egg yolks to saucepan.
Bring 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 filling evenly into crust. Cover with plastic wrap while you prepare the meringue.
To make the meringue: (To expedite, you may choose to make the meringue first so that it's ready to go once you've cooked the filling.} Using an electric mixer using the whisk attachment, beat egg whites with cream of tarter and vanilla until foamy. Slowly add sugar while beating. Beat until all sugar has been added and stiff glossy peaks, this will take several minutes. (Tip: The slower you add the sugar will help it dissolve and prevent a gritty texture.)
Remove plastic from pie. Spread meringue on top taking it completely to the crust. Bake at 350°F for 8-10 minutes until browned or broil for several minutes, turning for even browning. Don't walk away.
Chill for at least 4-6 hours before slicing.