Zest both lemons before cutting them. Use a zester or microplane and stay on the bright yellow outer peel — don't go deep enough to hit the white pith, which will give you a bitter taste. Set the zest aside.

Roll each lemon under your palm on the counter to help release the juices. Cut them in half and juice them. You want ¼ cup of fresh lemon juice — if your two lemons give you a little more than that, just use the quarter cup. Too much juice can keep the filling from setting up properly.

In a medium saucepan, whisk together the cornstarch, flour, and a pinch of salt until any lumps are broken up.

Add the water and sugar to the dry mixture and stir to combine. Place on the stove over medium-low heat, stirring constantly. You want to bring this to a low boil — be patient, it takes a while. You'll notice it gradually getting thicker as the cornstarch and flour do their work.

While the filling heats, lightly beat your 5 egg yolks in a separate bowl and set them nearby.

Once the mixture reaches a low boil and has thickened, remove it from the heat. Now we need to temper the egg yolks so they don't scramble. Slowly pour about a cup of the hot mixture into the beaten egg yolks, whisking constantly as you pour. This brings the eggs up to temperature gradually.

Pour the tempered egg mixture back into the saucepan with the rest of the filling. Return to the stove on low heat and cook for exactly 3 minutes, stirring constantly.

Remove from heat. Stir in the lemon juice and lemon zest. Then add the butter one tablespoon at a time, stirring until each piece melts before adding the next. The filling should look like a smooth, thick pudding.

Pour the filling into your baked pie crust and set aside while you make the meringue.

Add the 5 egg whites to the bowl of a stand mixer (or use a hand mixer). Add the cream of tartar, pinch of salt, and vanilla.

Begin beating on medium speed. Gradually add 1 cup of sugar while the mixer runs. This is more sugar than a typical meringue, but it balances the tartness of the lemon beautifully.

Increase speed to high and beat until glossy, stiff peaks form. The meringue will look shinier and more marshmallowy than a standard meringue — that's exactly what you want. It may look like it's not coming together at first, but keep going. It will get there.

Spoon the meringue over the warm filling, spreading it all the way to the edges of the crust to seal it. Use your spoon or spatula to create peaks and swirls on top.

Place the pie in a preheated 350°F oven. Bake for about 20 minutes. The meringue will turn a light, even golden brown — it won't develop the dark brown peaks you sometimes see with traditional meringue because of the extra sugar.

Remove from the oven and let the pie cool completely before slicing — this will take several hours. The filling needs that time to fully set up.