Preheat oven to 325ºF.: As your oven warms to 325ºF , you should notice a steady hum and gentle heat spreading through the chamber, which helps the meringue brown evenly without scorching. Preheating ensures that when you bake the assembled pie the exterior of the meringue develops that delicate tan while the interior remains soft. A common mistake is not preheating long enough, which leads to uneven browning, so wait until the oven registers the full temperature before baking.
In a medium saucepan, whisk together sugar, cornstarch, salt, egg yolks, milk and lemon juice. Bring to a boil over medium heat, continue whisking constantly for 1 minute. Remove from heat. Whisk in butter and lemon zest. Pour into cooked pie crust.: When these ingredients come together in the saucepan you will first hear a light simmer and then the mixture will thicken noticeably as the cornstarch activates. The aroma shifts from powdered to custardy, and you will feel the sauce coat the back of a spoon. Whisking constantly prevents lumps and keeps the texture smooth. After boiling for the short, specified time, removing from heat and whisking in the butter adds gloss and richness, while the lemon zest releases fragrant oils that lift the whole filling. One pitfall is overheating, which can scramble the egg yolks , so maintain medium heat and steady whisking to avoid curdling.
In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites and cream of tartar until foamy. Gradually add sugar, 1 tablespoon at a time, beating until stiff peaks form (about 4 minutes). Add vanilla, beat well. Spread meringue over pie filling.: As you begin whipping the egg whites with cream of tartar , the mixture will go from clear to opaque and then to glossy white foam; you will hear the mixer change tone as the structure builds. Adding the sugar slowly helps dissolve the crystals and prevents a grainy texture, and achieving stiff peaks means the meringue will hold lofty shapes when spread. The aroma will be sweet and clean, and the texture should feel satiny between your whisk and spatula. A common misstep is adding sugar too quickly, which can deflate the foam, so patience is essential here.
Bake for 20 to 25 minutes, until meringue is lightly browned. Remove from oven and cool on wire rack for an hour. Refrigerate until ready to serve.: As the pie bakes, the meringue will puff and the surface will go from glossy white to a warm, golden hue with subtle, toasted notes in the scent. You may hear a soft crackle as moisture escapes and the peaks set. Cooling on a wire rack allows air to circulate beneath the pie so condensation does not make the crust soggy, and refrigerating afterwards helps the filling finish setting to a sliceable consistency. Avoid overbaking which dries the meringue and causes excessive shrinkage; aim for a light brown color and touch the surface gently only once to check firmness.