In a double boiler, fill the bottom with a few inches of water and bring to a boil over high heat. Once boiling, reduce to a gentle simmer. In the top bowl, whisk together the lemon juice, lemon zest, sugar, salt, and egg yolks. Stir frequently so the egg yolks don’t curdle, 12-15 minutes, or until the mixture thickens and can coat the back of a spoon. The curd will continue to thicken off the heat. If the curd does not thicken within that time, increase the heat and give it 5-10 more minutes.

Remove from the heat and whisk in the butter, one pat at a time. Transfer to a jar, and press the top with a sheet of plastic wrap. Allow curd to cool before transferring and storing in the refrigerator for up to 3 weeks. You can also store the curd in the freezer for up to 6 months—just allow it to thaw completely before you’re ready to use.