Zest the lemons, then slice them in half and juice them into a measuring cup. Use a small colander or a lemon juicer to keep out any seeds.
Place the egg yolks, cornstarch, and salt in a large bowl. Whisk well to combine and set aside. (You will be adding more to this later, so make sure it's a large enough bowl.)
Add your lemon juice, granulated sugar, and lemon zest to a large, shallow sauté pan.
Stir constantly with a whisk and bring to a simmer over medium heat.
When it has reached a simmer, scoop about 1 cup of the lemon juice mixture and slowly add it into the egg yolk mixture while whisking. Add about 1 cup of liquid total.
Add the tempered egg mixture back into the lemon mixture while whisking constantly. Keep an eye on it and keep whisking, if you walk away for even a minute, the eggs can curdle.
Whisk constantly and cook over medium heat until desired thickness. I cook mine for about 2 minutes because I like thick lemon curd. Use a thermometer to check the temperature of the lemon curd.Removing the curd at 170ºF (76ºC)will yield a thinner consistency while removing at 180ºF (82ºC) will be thicker.
Add your butter in chunks to the lemon curd and whisk until the buter is melted and combined. Remove the lemon curd from the heat. It will continue to thicken as it cools.
Pour the finished lemon curd into a heat-proof jar or bowl. Cover the curd with plastic wrap so that it is touching the surface of the curd without any air bubbles in between, this will prevent a skin from forming on the top of the curd. Store it in the fridge for up to one week or freeze it for up to a year.