Preheat oven 350 degrees F. Whisk eggs, egg yolks, sugar, lemon juice, lemon zest and cornstarch. Cook over medium heat, stirring constantly until enough to coat back of spoon. Do not boil! Remove from heat and fold in sour cream. Pour into crust.
Meringue: Beat egg whites medium speed until foamy; add cornstarch and cream of tartar until soft peaks appear. Add sugar gradually and vanilla. Spoon meringue over filling, bringing it up to edges so pie doesn't bleed.
Bake until meringue is golden in color 12 -15 minutes. Sprinkle lemon zest on top if desired. Cool.