In a mixing bowl, combine brown sugar and lemon zest, rubbing them together to release the lemon oils. Beat in softened butter until fluffy, then incorporate the egg and vanilla extract. Gradually fold in flour, baking powder, and salt until well combined. Cover and chill for at least 30 minutes.

While the cookie dough chills, whisk together granulated sugar, fresh lemon juice, lemon zest, egg yolks, and a pinch of salt in a saucepan. Cook over medium heat until thickened, about 10 minutes. Strain the curd and mix in cubed butter until smooth. Allow to cool.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop rounded tablespoons of dough onto the sheet, spacing them apart. Bake for 10-12 minutes until edges are golden. Let cool on a wire rack.

Combine egg whites and granulated sugar in a clean saucepan. Heat gently while whisking to dissolve the sugar until it reaches 170°F (76°C). Transfer to a mixer and whip until stiff peaks form.

Spread a layer of lemon curd atop each cooled cookie and pipe the meringue on top. Use a culinary torch to lightly toast the meringue until golden brown. Let cool slightly before serving.