Position a rack in the middle of the oven. Heat the oven to 375°F.
Whisk the egg yolks in a medium mixing bowl and set aside.
Whisk togetherThis will eliminate any clumps in the cornstarch. cornstarch, sugar, and salt in a medium saucepan. Add the water, butter, lemon juice, and lemon zest. Bring mixture to a simmer, stirring gently, over medium heat. Remove from heat. One whiskful at a time, add the cornstarch mixture to the egg yolks and stir until you have added at least half of the cornstarch mixture.
Return the all of the egg-starch mixture to saucepan, turn heat down to low and cook, stirring gently, for one more minute. Pour the mixture into the pie crust.
Combine the egg whites and cream of tartar in the bowl of stand mixer with whisk attachment. Beat the egg whites until soft peaks form, and then gradually add the sugar and continue beating until stiff peaks form, about 2 minutes. Top the pie with the meringue while lemon filling is still hot. Be absolutely certain that you take the meringue all the way to the edges of the crust. You should not be able to see even a sliver of the lemon filling underneath.
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, this will take 4 to 6 hours. Your best bet is to cool overnight before cutting.