In a glass heat-proof bowl whisk together the 3 egg yolks, ⅔ cup granulated white sugar, ½ cup lemon juice, 1 tablespoons lemon zest , and ⅛ teaspoon salt. Place the bowl over a saucepan filled with 2 inches of simmering water. Make sure the bottom of the bowl doesn't touch the water.
Continue whisking as the lemon curd cooks. Constant whisking prevents the egg yolks from curdling. Keep whisking until the mixture thickens enough to coat the back of a spoon. Approximately 10 minutes.
Remove the bowl from the heat and whisk in the ¼ cup unsalted butter one tablespoon at a time. Let each piece melt partially before adding more.
(Optional) For a smoother texture press the lemon curd through a fine-mesh strainer to remove the lemon zest and any curdled egg.
Pour the finished lemon curd into a bowl or jar. Place a piece of plastic wrap directly on the surface of the curd to prevent a skin from forming.
The lemon curd will continue to thicken as it cools. Once the lemon curd has cooled to room temperature transfer it to the refrigerator.
Notes
When zesting the lemons, use only the yellow outer skin. Avoid the bitter white pith underneath it.Cook the curd over gentle heat, to avoid scrambling the egg yolks.Don't stop whisking! Constant whisking keeps the egg yolks from curdling into scrambled eggs.If you want a completely smooth curd, strain it through a fine-mesh strainer after cooking.Press a piece of plastic wrap directly on top of the hot lemon curd to prevent a skin from forming.Storage:
Lemon curd can be refrigerated in an airtight container for up to 10 days.
For longer storage, freeze the lemon curd in an airtight container for 3-6 months.
To thaw, place the frozen lemon curd in the refrigerator to thaw overnight.