In a medium mixing bowl, combine brown sugar and lemon zest, stirring to integrate. Add softened butter, beating for 2-3 minutes until fluffy and light. Next, incorporate the egg and vanilla extract, blending until well mixed. Gradually add flour, baking powder, and salt, mixing until just combined. Chill the dough in the refrigerator for at least 30 minutes to firm up.

While the dough chills, prepare the lemon curd. In a medium saucepan, whisk together sugar, lemon juice, lemon zest, egg yolks, and a pinch of salt. Cook over medium heat, stirring continuously until the mixture thickens and reaches about 170°F. Remove from heat, strain the curd through a sieve into a bowl with softened butter, and stir until smooth. Chill in the refrigerator until ready to use.

Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper. Once the dough has chilled, scoop out portions and place them onto the prepared sheets, allowing space for spreading. Bake for 10-12 minutes until the edges are golden and the centers are just set. Let cookies rest on the baking sheets for 5 minutes before transferring to wire racks to cool completely.

To create the meringue, combine egg whites and sugar in a small saucepan. Whisk constantly while heating over low-medium heat until the mixture reaches 170°F and the sugar dissolves. Transfer it to a stand mixer and beat on high speed until stiff peaks form. Gently fold in cornstarch and vanilla extract.

Once cookies have cooled, generously dollop the lemon curd onto each cookie. Carefully spoon the meringue over the lemon curd, swirling it for a decorative effect. Using a culinary torch, toast the meringue until golden brown.