Preheat oven to 350 F and place baked pie crust on a rimmed baking sheet
separate eggs and reserve yolks for filling and whites for meringue.
Lemon Filling
Whisk sugar cornstarch and salt in a saucepan then whisk in water until smooth
cook over medium heat whisking constantly until thick and bubbling
whisk yolks in a bowl then slowly whisk in a few spoonfuls of hot mixture to temper
pour tempered yolks back into saucepan and cook 1 minute whisking
remove from heat and whisk in butter lemon juice and lemon zest
pour hot filling into the prepared crust.
Meringue and Bake
Beat egg whites cream of tartar and salt to soft peaks then gradually beat in sugar until glossy stiff peaks form and beat in vanilla
spread meringue over hot filling sealing to crust edges
bake until meringue is lightly golden about 10 to 12 minutes
cool at room temperature 1 hour then chill at least 3 hours before slicing.
Notes
1. Seal meringue to the crust edge to prevent shrinking; Spread meringue on hot filling to reduce weeping; For clean slices wipe knife between cuts; Substitute 2 tbsp flour for cornstarch if needed but cook 1 to 2 minutes longer