In a mixing bowl, combine all-purpose flour, powdered sugar, and salt. Cut in the chilled unsalted butter until the mixture resembles coarse crumbs. Press this dough into the bottom of a lined baking tray. Chill while preheating the oven.

Preheat your oven to 350°F (175°C). Bake the cookie base for 15–18 minutes until lightly golden. Remove and let it cool slightly.

In a saucepan, whisk together lemon juice, granulated sugar, egg yolk, cornstarch, and lemon zest. Cook over medium heat, stirring until thickened (5–7 minutes). Pour over cooled base and bake for 10–12 minutes until set but slightly jiggly.

In a mixing bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add granulated sugar, continuing to beat until stiff peaks form.

Spread the meringue over the filling, creating peaks and ensuring it seals the edges of the cookie base. Bake at 200°F (90°C) for 30–35 minutes to dry out the meringue.

Turn off the oven and crack the door, allowing cookies to cool gradually for 15–20 minutes. Remove from the oven and let cool to room temperature before serving.