Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Mix brown sugar with lemon zest in a bowl, then beat in butter until fluffy. Add egg and vanilla extract to form a dough.

Gradually mix in flour, baking powder, and salt. Cover and chill for 30 minutes.

For the lemon curd, cook sugar, lemon juice, lemon zest, egg yolks, and salt over medium heat until thickened (7-10 minutes).

Strain the curd and mix with cooled butter until smooth; refrigerate to set.

Scoop dough onto the baking sheet and bake for 10-12 minutes until golden.

Cool on the baking sheet for 5 minutes, then transfer to a rack.

Whip egg whites and sugar over simmering water until warm, then whip until stiff peaks form.

Spoon lemon curd onto cooled cookies, top with meringue, and swirl decoratively.

Toast the meringue with a culinary torch until golden brown.

Serve immediately or store in an airtight container in the fridge.