Prep the crust: Blind-bake a 9-inch pie shell according to package or recipe directions until golden and crisp. Cool on a rack. Keep the oven at 350°F (175°C) for finishing later.

Zest and juice: Finely zest 1–2 lemons to get 1–2 tablespoons. Juice enough lemons to get about 3/4 cup. Strain the juice to remove seeds and pulp.

Start the lemon base: In a medium saucepan, whisk 1 cup sugar, 1/3 cup cornstarch, and a pinch of salt. Slowly whisk in 1 1/2 cups water until smooth. Cook over medium heat, whisking constantly, until the mixture thickens and gently bubbles, 5–7 minutes.

Temper the yolks: In a bowl, whisk 4 egg yolks. Slowly ladle in a cup of the hot mixture, whisking constantly. Pour the tempered yolks back into the saucepan and cook 1–2 minutes more, whisking, until glossy and thick.

Finish the filling: Remove from heat. Whisk in 4 tablespoons butter, the lemon juice, and zest until smooth. Taste and adjust lemon/sugar if needed. The filling should be tangy and bright.

Make the meringue: In a clean bowl, beat 4 egg whites with 1/2 teaspoon cream of tartar on medium speed until soft peaks form. Gradually add 1/2 cup sugar, a tablespoon at a time, then increase speed to medium-high and beat to glossy stiff peaks. Beat in 1 teaspoon vanilla if using.

Assemble while hot: Pour the hot lemon filling into the cooled crust. Immediately spoon meringue over the filling, spreading to the crust’s edges to seal. Create swirls on top with a spoon.

Bake to set: Bake at 350°F (175°C) for 12–15 minutes until the meringue is lightly browned. Avoid overbaking, which can cause weeping later.

Cool and set: Cool the pie at room temperature away from drafts for 1–2 hours. Then chill for at least 3 hours before slicing. Use a hot, dry knife for clean slices.