In a food processor, combine flour, butter, sugar, and salt. Pulse until the mixture resembles coarse crumbs.
Slowly add the ice water, pulsing just until the dough comes together when squeezed. Do not overmix.
Turn the dough out onto a lightly floured surface, form it into a flattened disk, wrap in plastic, and refrigerate for at least 2 hours.
Preheat your oven to 400°F (200°C). Roll out the chilled dough to about ¼ inch thick, making it 1 inch larger than your 9-inch pie dish.
Carefully transfer the dough to the pie dish. Trim and crimp the edges as desired. Prick the bottom with a fork a few times to prevent bubbling.
Line the crust with foil, filling it with pie weights or dried beans. Bake for 15 minutes.
Remove the foil and weights, then bake for another 5-10 minutes, or until the crust is golden brown. Set aside to cool completely. Reduce oven temperature to 350°F (175°C).
In a large saucepan, combine sugar, cornstarch, salt, water, and fresh lemon juice. Whisk until smooth before turning on the heat.
Cook over low heat, stirring constantly, until the mixture begins to thicken. It will happen quickly.
In a separate bowl, whisk the beaten egg yolks. Slowly drizzle about half of the hot lemon mixture into the egg yolks, whisking constantly to temper them.
Pour the tempered egg yolk mixture back into the saucepan with the remaining lemon mixture. Continue to cook, stirring, until it simmers and thickens further, about 1 minute.
Remove from heat and stir in the cubed butter and lemon zest (if using) until melted and fully incorporated. Pour the hot filling into your cooled pie crust immediately.
In a clean, dry bowl of an electric mixer, beat the room temperature egg whites on high speed until they are very frothy.
Add the vanilla extract and cream of tartar, continuing to beat on high speed.
Gradually add the sugar, one tablespoon at a time, beating until the meringue is stiff, glossy, and holds firm peaks, about 5-7 minutes.
Carefully spoon the meringue onto the hot lemon filling. Start at the edges of the crust and spread the meringue to completely seal the filling. Create decorative peaks with the back of a spoon.
Bake at 350°F (175°C) for 5-10 minutes, or until the meringue tips are beautifully golden brown.
Let the pie cool on a wire rack for at least 1 hour, then refrigerate for a minimum of 3 hours, or preferably overnight, before slicing and serving. This chilling time is crucial for the filling to set.