Bring a saucepan of water to a gentle simmer and set a glass or metal bowl over the top to create a double boiler. In the bowl, whisk together eggs, yolks, sugar, butter, lemon zest, and lemon juice.
Cook over the simmering water, stirring frequently, until very thick, about 20 to 25 minutes (the curd must reach 170 degrees on a thermometer to thicken).
Strain through a fine-mesh strainer or cheesecloth, then cover and chill completely.
Notes
Lemons: Always zest your lemons before you cut them open to juice. It’s much harder to zest half of a lemon! Here's How to Zest a Lemon five ways.
Yield: This recipe makes 2 1/2 cups of lemon curd.
Storage: Store lemon curd covered in the refrigerator for up to 1 week.