In a large mixing bowl, combine all-purpose flour, powdered sugar, and salt. Cut in the unsalted butter until mixture resembles coarse crumbs.

Incorporate the egg yolk and freshly squeezed lemon juice, mixing until a soft dough forms. This will take about 2-3 minutes.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Roll the dough into small balls, about 1 inch in diameter.

Place the balls on the prepared baking sheet, leaving space between each. You should have about 24 dough balls.

Bake the cookies for around 12-15 minutes until set but not browned. Allow them to cool on the baking sheet for a few minutes.

In a medium saucepan, whisk together the egg yolk, granulated sugar, cornstarch, lemon juice, and zest. Cook over medium heat until thickened, about 5-7 minutes.

In a clean mixing bowl, beat the egg whites with cream of tartar until soft peaks form. Gradually add granulated sugar until glossy and stiff peaks form.

Once cookies have cooled, dollop the thick lemon filling onto each cookie.

Spoon the meringue over the lemon filling, creating peaks and swirls. Bake at 200°F (90°C) for about 10 minutes until meringue is lightly browned.

Remove the cookies from the oven and allow to cool on a wire rack. Store leftovers in an airtight container at room temperature.