In a mixing bowl, combine all-purpose flour, powdered sugar, and salt. Cut in chilled unsalted butter until the mixture resembles coarse crumbs. Add egg yolk and lemon juice, mixing until just incorporated. Set aside while preheating the oven.

Preheat your oven to 200°F (90°C). Form the dough into small balls, about the size of a walnut. Place on a parchment-lined baking sheet, spacing them about an inch apart. Bake for 15-20 minutes or until set but not browned.

While baking, prepare the lemon filling by whisking together lemon juice, granulated sugar, cornstarch, and lemon zest in a saucepan. Cook over medium heat until thickened, about 5 minutes, then remove from heat to cool slightly.

In a clean bowl, whip egg whites and cream of tartar until soft peaks form, about 3-5 minutes. Gradually add sugar, continuing to whip until stiff peaks form and meringue is glossy.

Once cookies are cooled, spoon lemon filling onto each, then pipe or spread meringue on top, covering filling completely. Swirl decoratively for an attractive finish.

Return cookies to the oven and bake at 200°F (90°C) for about 10 minutes until meringue is lightly browned. Avoid over-browning; remove from oven when golden.

Let cool in the oven with the door slightly ajar for about 30 minutes. Once cooled, transfer to a serving platter.