01 -
Preheat the oven to 350°F (175°C). Set the baked pie crust on a rimmed baking sheet so it is easy to move once filled.
02 -
In a medium saucepan, whisk together the sugar, cornstarch, flour, salt, water, lemon juice, and lemon zest until smooth. Cook over medium heat, whisking often, until the mixture comes to a bubbling boil and thickens.
03 -
Slowly whisk about 1/2 cup of the hot lemon mixture into the beaten egg yolks. Pour the warmed yolks back into the saucepan while whisking constantly.
04 -
Cook the filling for 1 minute more, whisking constantly, until very thick and glossy. Remove from the heat, stir in the butter until melted, and immediately pour the hot filling into the baked crust.
05 -
Beat the egg whites and cream of tartar in a clean bowl until foamy. Gradually add the sugar, 1 tablespoon at a time, then beat until glossy stiff peaks form. Beat in the vanilla briefly.
06 -
Spoon the meringue over the hot lemon filling. Spread it all the way to the crust edge to seal, then make soft swirls with the back of a spoon.
07 -
Bake for 10-15 minutes, until the meringue is golden on the peaks. Cool at room temperature for at least 3 hours before slicing, then refrigerate leftovers loosely covered.