In a saucepan over medium-low heat, whisk together the fresh lemon juice, sugar, eggs, egg yolks, and salt. Whisk constantly for 5–8 minutes until the mixture thickens beautifully.
Once thickened, remove from heat and whisk in the unsalted butter until completely melted and glossy.
Pour the curd through a fine-mesh strainer set over a bowl to catch any small lumps. Cover the bowl tightly with plastic wrap (press it directly onto the surface of the curd to prevent a skin from forming) and let it cool. Pop it in the fridge while you bake the cookies.
Add the egg and fresh lemon juice, mixing until completely combined.
Pour in the flour and baking powder. Mix on low speed just until the flour disappears. Do not overmix, or the cookies will turn out dense and dry.
Scoop the dough into 2-tablespoon balls and place them on a parchment paper-lined baking sheet. Roll into smooth balls with clean hands.
Dip a teaspoon or a tablespoon measuring spoon into a bit of flour, then press a deep indent into the center of each dough ball. Bake in a preheated 180°C (350°F) oven for 10–12 minutes until the edges are set.
If the cookies puffed up in the oven and lost their wells, immediately use the back of a small spoon to gently press the indents back down while they are still piping hot.
Then, use a large round glass or biscuit cutter to gently swirl around the warm cookies to form perfect circles. Let them cool completely.
Make the Swiss meringue
Combine the egg whites, sugar, and a pinch of salt in a heatproof bowl. Place the bowl over a pot of simmering water (make sure the bottom of the bowl doesn't touch the water).
Whisk constantly until the sugar is completely dissolved and the mixture reaches 71°C (160°F), or feels completely smooth and non-grainy when rubbed between your fingers.
Remove from heat and transfer to a stand mixer fitted with a whisk attachment. Beat on high speed for 5–7 minutes until stiff, glossy peaks form, and the bowl feels cool to the touch.
Assembly
Spoon or pipe 1–2 teaspoons of chilled lemon curd into the center well of each cooled cookie. Pipe a generous swirl of Swiss meringue right on top of the curd, then toast beautifully with a kitchen torch right before serving.
Notes
Don't Overmix the Dough: Mix the dry ingredients just until combined. Overworking the dough develops gluten, which makes the cookies tough instead of melt-in-your-mouth tender.
Keep the Curd Moving:Whisk the lemon curd constantly while it cooks over medium-low heat. If you leave it untended, the eggs will scramble and ruin the texture!
Create a Deep Well: Make a generous indentation in the raw dough balls using a floured spoon. The well will puff up slightly in the oven, so you want it deep enough to hold plenty of lemon curd.
Underbake Slightly: Take the cookies out when the edges are just set but the centers still look a little soft. They will firm up as they cool on the baking sheet, staying perfectly tender.
Ensure Your Meringue Bowl is Grease-Free: Even a single drop of fat or egg yolk will stop egg whites from whipping into stiff peaks. For insurance, wipe your mixing bowl and beaters down with a little lemon juice or white vinegar before starting.
Cool Completely Before Filling: If you add the lemon curd to warm cookies, it will melt, become runny, and make the cookie base soggy.