MIX FILLING BASE: In a medium saucepan, whisk together 1 cup granulated sugar and ¼ cup cornstarch. Gradually whisk in 1 ½ cups cold water until smooth.
ADD YOLKS: Whisk the egg yolks lightly, then add them to the saucepan. Stir until fully combined. Set aside.
PREP LEMON: Measure the lemon juice and lemon zest and set aside.
START MERINGUE: In a large, clean mixing bowl or stand mixer fitted with the whisk attachment, beat the egg whites until foamy. Add the cream of tartar and continue beating on high speed.
ADD SUGAR TO MERINGUE: With the mixer running, gradually add the ½ cup granulated sugar. Beat until stiff, glossy peaks form.
COOK FILLING: While the meringue whips, place the saucepan over medium heat. Whisk constantly until the mixture comes to a low boil. Continue whisking and boil gently for 1 minute.
FINISH FILLING: Remove from heat and whisk in the lemon juice, lemon zest, and butter until smooth.
ASSEMBLE PIE: Immediately pour the hot filling into the pre-baked pie crust. Spoon the meringue over the hot filling, spreading it all the way to the crust edges to seal. Create swirls and peaks with a spoon or spatula.
BAKE: Bake for about 15 minutes, until the meringue is lightly browned.
COOL: Remove from the oven and allow the pie to cool completely before slicing and serving.
Notes
Crust Tip: The pie crust must be fully baked before adding the filling. Follow blind-baking instructions on the crust packaging or recipe.Meringue Tip: Always spread meringue onto hot filling and seal it to the crust edge to help prevent weeping or shrinking.Egg White Tip: Room-temperature egg whites whip best. Make sure the bowl and whisk are completely clean and free of any yolk.Cooling: Allow the pie to cool completely before slicing so the filling sets properly.Storage: Cover loosely with foil and refrigerate for up to 2 days.