In a medium saucepan, add the sugar and lemon zest. Rub the zest into the sugar for 30 seconds. Add the lemon juice, egg, yolks, and salt. Whisk to combine. Place pot over medium heat and whisk continuously until thickened, 5-7 minutes. Transfer to a heat-proof bowl. Stir in the butter and vanilla. Cover with plastic wrap and refrigerate.

In a large bowl, beat the butter and sugar until light and fluffy. Beat in the egg and egg yolk. Add the vanilla, baking powder, and salt; mix well. Gradually add the flour and mix on low until combined.

Preheat oven to 350°F. Grease a flat-bottomed measuring cup. Stir the refrigerated lemon curd. Line two half-sheet trays with parchment paper. Using a quarter-cup ice cream scoop, place 6-7 cookie dough balls on the trays and flatten slightly.

Bake trays for 10-12 minutes until edges are golden. Reduce oven heat to 300°F. Remove the tray with the more-cooked cookies. Press down in the center of each cookie to create a ¼” deep divot. Spoon 1-2 tbsp lemon curd into each divot. Return both trays to the oven and bake for another 5-8 minutes to set the curd.

Fill a medium saucepan with 1” of water and bring to a simmer. In a mixing bowl, combine egg whites, granulated sugar, and salt. Set over simmering water and whisk until warm. Attach bowl to a stand mixer and whisk on high until cool and peaks form, about 3-4 minutes. Add powdered sugar and vanilla; whisk until incorporated. Pipe meringue on top of lemon curd. Use a kitchen torch to caramelize the meringue.