Preheat the oven to 450 degrees F. For the crust, whisk the flour, sugar, salt and baking powder in a 9-inch pie dish. Whisk the milk and olive oil in a measuring cup and add the liquid to the pie dish. Stir with a fork to blend. When the dough is mixed, press it evenly along the bottom and up the sides of the dish to form a crust. (It will feel soggy.) Crimp the edges with two fingers.
Use a fork to poke holes in the bottom of the crust and where it meets the sides, so it won't puff up while baking. Bake for 10-12 minutes or until golden. (If you see your crust puffing in the oven, just open the door and poke more holes.) Let the crust cool on a wire rack.
While the crust is baking, separate your eggs, placing the yolks in a large mixing bowl and the whites in a medium mixing bowl. Beat your yolks slightly with a fork. Set the yolks and whites aside, letting them come to room temperature.
For the filling, rinse and dry two lemons. Zest the lemons until you have 2 teaspoons. Cut your lemons in half (you will probably need half of a third lemon) and squeeze them until you have 1/3 cup of lemon juice. Set it next to your stove.
In a medium, heavy-bottomed saucepan (I use my stainless steel one) whisk together your sugar, flour, corn starch and salt. Gradually whisk in the water. Make sure your corn starch mixture is fully incorporated before placing the pan over heat. Use a silicone spatula for gently stirring from this point forward. Place the pan over medium-high heat and gently stir until thickened and bubbly. Reduce the heat. Cook and stir gently for two more minutes, and remove the pan from the heat.
To temper your egg yolks so they don't scramble, gradually mix in one cup of the hot filling into your bowl of yolks. Then, add the yolk mixture to your pot with the rest of the filling. Stir gently over medium-high heat and bring to a gentle boil. (Lower the heat if needed.) Cook and stir gently two minutes more. Remove from the heat. Your filling should be very thick.
Carefully stir in the butter and lemon zest. Gradually stir in the lemon juice (gently!), making sure the liquid is fully incorporated before adding more. Stir until there is no liquid pooling at the sides of your pan.
Pour the hot filling into your pie crust. Freeze the filling and crust for 10-20 minutes, until the filling is firm to the touch and set.
While the filling is chilling in the freezer, make your meringue. Add the lemon extract and cream of tartar to your egg whites in the medium mixing bowl. Using clean beaters, beat the egg whites with an electric mixer on medium speed until soft peaks form. (The tips will curl.) Gradually add your sugar, one tablespoon at a time, beating at high speed until the sugar is dissolved and stiff, glossy peaks form. (When you turn off your mixer and lift up your beaters, the peaks should stand up firmly without falling over.)
Spoon the meringue over your chilled filling, starting at the edges of the filling, where it meets the crust. Completely cover the filling and seal it by pressing down slightly at the edges. When all of your meringue is spooned on top, you can make decorative swirls using the back of your spoon to smooth and lift the peaks. Use a kitchen torch (or a blow torch from your garage) to brown the meringue.
Let the pie sit at room temperature for at least half an hour before slicing it, making sure the bottom of the pie is cool first. Store leftover pie in the refrigerator. You can leave it uncovered so as not to mess up the meringue.