Prepare and Chill the Pie Crust: Begin by preparing your homemade pie dough according to your favorite recipe or pre-made dough. Once formed, wrap it tightly and refrigerate for at least 2 hours to firm up, which will make rolling easier and prevent shrinking during baking.
Blind Bake the Pie Shell: Preheat your oven to 375°F (190°C) and position the rack in the lowest slot. Roll out the chilled dough on a lightly floured surface and place it into a 9-inch pie dish, trimming and crimping the edges as desired. Line the crust with parchment paper and fill with pie weights or dried beans to prevent bubbling. Bake for about 15-20 minutes until the edges start to set and the bottom looks dry. Remove weights and parchment, then bake an additional 5 minutes until golden. Lower the oven temperature to 350°F (177°C).
Prepare the Lemon Filling Base: In a medium saucepan, combine water, sugar, cornstarch, salt, lemon juice, and lemon zest. Whisk continuously over medium heat until the mixture thickens and begins to bubble, about 6 minutes. Remove from heat temporarily.
Temper the Egg Yolks: In a separate bowl, whisk the egg yolks until smooth. Slowly add a few spoonfuls of the hot lemon mixture into the yolks while whisking constantly to gradually raise their temperature without scrambling. Then, pour this yolk mixture back into the saucepan with the remaining lemon mixture. Return the pan to medium heat and cook, stirring constantly, until the filling thickens further and bubbles burst on the surface. Remove from heat and stir in the softened butter until melted and smooth.
Fill the Pie Shell: Pour the warm lemon filling into the pre-baked pie crust, spreading evenly. Set aside while preparing the meringue, ensuring the filling stays warm to help it bond with the topping.
Whip the Meringue: Using a handheld or stand mixer fitted with a whisk attachment, beat the egg whites and cream of tartar on medium speed for 1 minute. Increase to high speed and continue beating until soft peaks form, about 4 minutes. Gradually add sugar and salt while continuing to whip until the mixture is glossy and forms stiff peaks, about 2 more minutes.
Top and Seal the Pie: Spread the meringue evenly over the lemon filling, making sure to seal the edges by pressing it onto the crust. For a decorative touch, use the back of a spoon to create small peaks.
Bake the Meringue: Bake the pie on the lowest oven rack at 350°F (177°C) for 20-25 minutes, or until the meringue is golden brown. If it starts browning too fast, tent the top loosely with foil to prevent burning.
Cool and Chill: Remove the pie from the oven and place it on a cooling rack. Let it cool to room temperature for about an hour, then refrigerate for at least 4 hours to fully set before serving.