In a medium saucepan, whisk the egg yolks, whole egg, and sugar together until just combined. Whisk in the lemon juice, zest, and salt. Add the butter and cook over medium-low heat, stirring constantly, until the mixture has thickened, about 4-8 minutes minutes.

Immediately strain the curd through a fine-mesh strainer into a medium-size bowl. Cover with plastic wrap and place in the refrigerator to cool completely.

Preheat oven to 350ºF degrees. Line a 12-count muffin pan with cupcake liners.

In a mixing bowl, whisk together the flour, baking powder, baking soda, and salt until combined; set aside.

Pour the dry ingredients into the wet ingredients and mix until combined. Add the milk, lemon juice, and zest, then mix until just combined, scraping down the sides and bottom of the bowl as needed.

Fill cupcake liners 2/3 of the way full. Bake for 16-18 minutes or until a toothpick inserted in the center comes out clean. Allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

Place the egg whites into the bowl of a stand mixer fitted with a whisk attachment (or large mixing bowl with a beater), and whisk on high speed until stiff peaks form.

Pour the sugar and water into a small saucepan, and cook over medium heat, without stirring, until it reaches 240ºF. This usually takes about 5-6 minutes from the time it begins to boil. Immediately pour the hot sugar syrup in a slow, steady stream into the whipped egg whites, with the mixer running on medium speed; try to avoid hitting the whisk attachment.