Preheat oven to 350°F. Line a baking sheet with parchment paper.

Cream the butter and sugar in a large bowl until light and fluffy, 2 to 3 minutes.

Beat in the egg, vanilla, lemon zest, and lemon juice until smooth.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.

Add the dry ingredients to the wet ingredients and mix until just combined.

Scoop about 1½ tablespoons of dough per cookie and roll into balls. Place on the prepared baking sheet.

Bake for 10 to 12 minutes, until edges are just golden and centers are set. Immediately use the back of a teaspoon to press a small well into the center of each cookie.

Let cool completely on the tray before filling.

In a saucepan, whisk together the egg yolks, sugar, cornstarch, water, and lemon juice.

Cook over medium heat, whisking constantly, until thick and glossy, about 5 to 7 minutes.

Remove from heat and stir in the butter and lemon zest.

Let cool until slightly warm, then spoon about 1 teaspoon into the well of each cookie. Chill while preparing the meringue.

In a stand mixer, whip the egg whites with cream of tartar until stiff peaks form. If unsure of your mixer's speed, start whipping before the syrup is ready and let the whites hold at stiff peaks until needed.

Combine the sugar and water in a saucepan over medium heat. Cook until the syrup reaches 244°F, checking with a candy thermometer or by dropping a small amount into cold water to test for a soft ball.

With the mixer running on high, slowly pour the hot syrup into the whipped egg whites in a thin, steady stream.

Continue beating until the meringue is glossy, thick, and holds firm peaks, and the bowl feels close to room temperature.

Spoon or pipe the meringue over each lemon-filled cookie.

Optional: torch lightly for color, or bake at 400°F for 2 to 3 minutes to lightly brown the tips.

Cool completely before serving.