In a large mixing bowl, cream together brown sugar, lemon zest, and softened butter until fluffy, about 3-4 minutes. Beat in an egg and a splash of vanilla extract, then gradually mix in the dry ingredients, including flour, baking powder, and salt. Chill the dough in the refrigerator for 30 minutes.

In a saucepan, combine sugar, freshly squeezed lemon juice, zest, egg yolks, and a pinch of salt. Cook over medium heat, stirring constantly until the mixture thickens, reaching a temperature of 170°F. Strain the lemon curd through a fine sieve into a bowl to remove lumps, then cool completely.

Preheat your oven to 350°F and line a baking sheet with parchment paper. Scoop chilled cookie dough into rounded tablespoons and place them on the prepared sheet. Bake for 10-12 minutes until edges are golden. Cool on the baking sheet.

In a heatproof bowl, combine egg whites and sugar, then set it over a saucepan of simmering water. Whisk constantly until the mixture reaches 170°F. Remove from heat and whip until stiff peaks form, about 5-6 minutes.

Spread cooled lemon curd over each cookie, then dollop or pipe the fluffy meringue on top. Toast the meringue lightly until golden brown with a kitchen torch.