Into a saucepan over medium-low heat, add the lemon juice, sugar, eggs, egg yolks, salt, and lemon zest, and mix with a whisk constantly.
Keep whisking until the mixture thickens for about 5-8 minutes. When the mixture thickens, add the butter and mix with a whisk until the butter is melted.
Transfer the mixture to a fine-mesh strainer set over a bowl and strain the curd from any unnecessary lumps. This will give you a perfect and smooth filling.
Cover the bowl tightly with plastic wrap and let the mixture cool for about 20-30 minutes at room temperature and then in the fridge while you bake the crust.
Add the flour and baking powder, and mix until just combined. Don't overmix the cookie dough for moist cookies and not dry cookies.
Scoop cookie dough balls with a 2-tablespoon cookie scoop and place them on a parchment paper-lined baking sheet (on two separate baking sheets). Roll each cookie dough into a ball using your clean hands.
Use a tablespoon or a teaspoon to create indents in each cookie dough ball. Dip the tablespoon in flour to prevent the spoon from sticking to the cookie dough.
Bake the cookies in a 180°C (350°F) preheated oven for 10-12 minutes. Use a cup or a large biscuit cutter to swirl around the cookies while they are warm to create perfect circles.
Make the Swiss meringue
Start by combining egg whites, salt, and sugar in a heatproof bowl. Place the bowl over a pot of simmering water, creating a double boiler.
Whisk the mixture constantly while heating until it reaches 160°F (71°C) or feels smooth and not grainy when rubbed between your fingers.
Once the desired temperature is reached, remove the bowl from heat and transfer the mixture to a stand mixer fitted with a whisk attachment.
Beat on high speed for about 5-7 minutes, or until stiff, glossy peaks form and the meringue has cooled to room temperature.
Assembly
Transfer the lemon curd into a piping bag and cut a small slit. Pipe 1-2 teaspoon of lemon curd into the center of each one. Pipe the meringue onto the tops of the cookies, then toast with a kitchen torch.
Notes
Use room temperature ingredients. It is important that you use room-temperature ingredients like eggs and butter. This step ensures that your mixture mixes into a homogenous mixture and bakes perfectly.
Don’t overmix the cookie dough, you don’t want to develop the gluten in the flour. This step ensures you have moist cookies and not dense and dry cookies.
Use fresh lemon juice and zest. This gives the cookies and curd a bright, natural lemon flavor that really shines.
Don't let the lemon curd come to a boil! This will cook the eggs and leave you with scrambled eggs. Make sure that the lemon curd mixture cooks over medium-low heat.
Use a cookie scoop. It helps make all the cookies the same size and shape for even baking.
Make them round. Right after baking, use a round cookie cutter or a spoon to gently nudge the edges into perfect circles while they’re still warm.
Underbake slightly. Take the cookies out when the edges are just set and the centers still look a little soft. they’ll finish baking as they cool and stay super tender.
Make sure your bowl is grease-free. Even a little fat can stop meringue from whipping. Avoid egg yolks in the whites, and for extra insurance, wipe your bowl with a bit of lemon juice before whipping.
Create a deep well. Use the back of a spoon which has been dipped in flour to prevent sticking to make a deep indentation in each cookie to hold plenty of filling.
Measure your flour correctly! When measuring your flour, avoid scooping it with a measuring cup. Instead, stir the flour with a spoon to create as many air pockets as possible, and spoon it into your measuring cup. Finally, level it with a back of a knife.
Line your sheet pan with parchment paper or a silicone mat! This step helps the cookies to bake evenly, and it makes sure that your brown butter chocolate chunk cookies release easily from the pan.
Cool completely before filling. Adding lemon curd to warm cookies can make them soggy or cause the curd to spread too much.