½cup(113g) unsalted buttersoftened to room temperature and cut into 8 pieces2
Fill the bottom pot of a double boiler with 3-4" of water and turn heat to high. Bring the water to a boil, then reduce to a simmer. You may also use a pot with a heatproof bowl3 on top instead of a double boiler. Prepare the water the same.
Combine the egg yolks, sugar, lemon zest, lemon juice, and salt in the top pot of the double boiler. Use a silicone whisk3 to continuously whisk the mixture as it cooks. It is important to keep the mixture moving so the eggs don't cook or curdle. Continue whisking the mixture until it becomes thick and foamy, approximately 10 minutes. If you want to check the temperature of the curd with an instant read thermometer to be safe, it should reach 170ºF (77ºC).
Remove double boiler or top bowl from heat and whisk in butter pieces 2 at a time, adding the next pieces after previous pieces have mostly melted.
When all of the butter has melted, place a piece of plastic wrap directly on top of lemon curd, making sure the plastic wrap is touching the surface of the curd. Allow lemon curd to cool in the refrigerator (it will thicken as it cools), about 2 hours. Remove plastic wrap when you are ready to use it. Leftover lemon curd stays fresh in the refrigerator, covered tightly, up to 1 month, or in the freezer, up to 1 year. Thaw in the refrigerator overnight.
Fresh lemon juice:do not use bottled lemon juice. Use freshly squeezed lemon juice from the lemons that you zest (3 lemons will give you a little more than ½ cup of juice).
Unsalted butter: you may use salted butter. Do not add additional salt.
Bowl and whisk: do not use a metal bowl or a metal whisk, or you run the risk of your lemon curd tasting metallic.