Whisk egg yolks in medium size mixing bowl and set aside.
In a medium saucepan, combine sugar, cornstarch, salt, and water. Whisk to combine. Turn heat on medium and, stirring frequently, bring mixture to a boil.
Boil for 1 minute. Remove from heat and gradually, a whisk-full at a time, add hot mixture to egg yolks and stir until you have added at least half of the cornstarch mixture. Once incorporated, add the remaining mixture.
Return egg mixture to saucepan, turn heat down to low and cook, stirring constantly for 1 more minute. Remove from heat and gently stir in butter, lemon juice, and zest until well combined.
To make meringue: Place egg whites and cream of tartar in the bowl of a stand mixer fitted with the whisk attachment. Beat egg whites until soft peaks form and then gradually add sugar and continue beating until stiff peaks form, approximately 1 to 2 minutes.
Pour lemon mixture into pre-cooked pie shell and top with meringue. Make sure meringue completely covers filling and that it goes right up to the edge of the crust.
Adjust the oven rack to the middle position. Preheat oven to 375 degrees.
Bake for 10 to 12 minutes or until meringue is golden. Remove from oven and cool on a wire rack. Make sure pie is cooled completely before slicing.