In a medium saucepan, add the sugar and lemon zest. Rub the zest into the sugar with your fingertips for 30 seconds. Add the lemon juice, egg, yolks, and salt. Whisk to combine. Place pot over medium heat and whisk continuously until the lemon curd thickens, 5-7 minutes. Dump lemon curd into a heatproof bowl. Stir in the butter and vanilla. Cover with plastic wrap ensuring the plastic touches the top of the curd and refrigerate until ready to use.

Add the butter and sugar to a large bowl. Beat with a hand mixer on medium-high speed until light and fluffy. Beat in the egg and egg yolk until combined. Beat in the vanilla, baking powder, and salt. Add the flour and beat on low until just combined.

Preheat oven to 350?F with a rack in the upper and lower thirds. Grease a 1/3 cup flat-bottomed measuring cup and set aside. Remove lemon curd from the refrigerator and stir it with a spoon. Line two half-sheet trays with parchment paper. Using a quarter-cup ice cream scoop, plop 6-7 cookie dough balls evenly spaced on the trays. Flatten the balls slightly.

Bake both trays for 10-12 minutes, or until the cookies are picking up color around the edges. Reduce oven heat to 300?F. Remove the tray with the more-cooked cookies from the oven, leaving the other one in the oven. Using the prepared measuring cup, press down into the center of each cookie, creating a ~1/4? deep divot. Spoon 1-2 tbsp lemon curd into each divot. Return the tray to the oven and repeat with the other tray. Bake for another 5-8 minutes to set the lemon curd. Remove trays from the oven and let cool completely.

Set a medium saucepan with 1? of water over medium-high heat and bring to a simmer. In the bowl of a stand mixer, add the egg whites, granulated sugar, and salt. Set over the simmering water and whisk continuously until the egg whites are very warm and not grainy. Attach bowl to the stand mixer and turn to high speed until the meringue cools down and holds peaks, about 3-4 minutes. Add the powdered sugar and vanilla and whisk until incorporated. Transfer meringue to a piping bag. Pipe meringue on top of the lemon curd in a desired design. Use a kitchen torch to caramelize the meringue.