Fill a saucepan halfway with water and let it come to a boil.
Meanwhile, take a heat-proof bowl that fits at the top of your saucepan. Add the egg yolks (4), lemon zest (15 gs), fresh lemon juice (100 ml), and sugar (140 gs) into the bowl and give it a quick whisk.
4 egg yolks, 15 gs lemon zest, 100 ml fresh lemon juice, 140 gs granulated white sugar
Once the water in the saucepan has come to a boil, reduce the heat and bring the water to a simmer. Then, place the heat-proof bowl with the lemon curd mixture on top of the saucepan. Make sure that the bottom of the bowl is not touching the water.
As soon as you place the bowl on the saucepan, start whisking the mixture and keep whisking continuously to stop the eggs from scrambling from the hear. Keep whisking until the mixture thickens and coats the back of a spoon, about 10-15 minutes.
Once thickened, take the lemon curd off the heat and immediately add the softened butter (90 gs) and whisk until it's melted incorporated into the mixture.
90 gs unsalted butter, softened
Cover the lemon curd with a cling film while you chill it in the refrigerator. Make sure that the cling film is touching the surface of the lemon curd to stop a film from forming on top of the curd. Let the curd chill for 1 hour. Give it a quick whisk before using to smooth out any lumps.