To prepare the dough, in a large bowl whisk together the flour, sugar and salt. In a separate bowl whisk together the egg yolk, vanilla and ice water. Slowly pour into the flour mixture, mixing until combined. Cut the butter into the mixture with a pastry blender until pea-size clumps form. Knead the dough into a ball, then pat flat and wrap with plastic wrap. Refrigerate for 1 hour.
After the dough has chilled, roll it out into a circle approximately ⅛" thick. Place into a 9-inch pie dish. Trim the dough so that it sticks up at least ½ inch above the rim, then fold over and crimp with your fingers or a fork. Using a fork lightly prick the bottom of the crust, taking care not to go all the way through. Place a piece of aluminum foil or parchment paper over top and pour in pie weights (see NOTES if you don't have weights). Place in the oven and bake at 325 degrees for 20 minutes, remove the foil and weights and bake for 5 more minutes or until pale golden brown. Allow to cool while preparing the filling.
To make the filling, in a medium saucepan, whisk together the sugar, flour, cornstarch, and salt. Stir in the milk, water, lemon juice and lemon zest. Cook over medium-high heat, stirring frequently, until mixture comes to a boil, then stir in butter. Place egg yolks in a small bowl and gradually whisk in ½ cup of hot sugar mixture. Whisk egg yolk mixture back into remaining sugar mixture. Bring to a boil and continue to cook while stirring constantly until thick.
Meanwhile, in the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites and vanilla for 1 minute on medium-high speed until frothy, then gradually beat in the cream of tartar and sugar, 1 tablespoon at a time, until the sugar is fully incorporated. Turn speed up to medium-high beating until the mixture forms soft peaks. Add the cornstarch mixture, 1 tablespoon at a time, while the mixture is still beating. Continue to beat until stiff peaks, about 1-2 minutes.
Remove filling from heat, whisk in the vanilla extract, then pour into the baked pastry shell. Spread the meringue on top of the HOT filling, spreading all the way to the crust. You want to make sure it touches the crust so that it seals the pie and prevents weeping. Place in the oven and bake the pie until the meringue is lightly browned, about 15-20 minutes. Transfer to a wire rack and allow to cool until room temperature.
-If you don't own pie weights, simply fill the foil-lined crust with rice or beans. You can store them and reuse them time and time again!
-Meringues can be very difficult! Make sure your egg whites are a room temperature, that your bowl is clean and there isn't a trace of fat in it (from butter or egg yolk), SLOWLY add the sugar in and take care to spread over a HOT filling and push it all the way to the edges of the crust so that it seals the pie.