Place sugar in a medium-sized pan over a pot of simmering water in a double-boiler. Gradually whisk in lemon juice. (Do NOT substitute bottled lemon juice for fresh in this recipe. It’s just not the same.)
Add eggs and egg yolks, continuing to whisk steadily. Whisk over medium to medium-high heat, 10 to 15 minutes, until mixture thickens. (If it’s not thickening, increase the heat, being careful to continue whisking. I recommend using a double-boiler especially if you’re not using the cornstarch.) It should be about the consistency of hollandaise sauce. Add butter, whisking to blend. Cook, continuing to whisk, one to two more minutes. Remove pan from heat. Stir in lemon zest.
Transfer to a bowl and place plastic wrap over it, pressing over the top to prevent a skin from forming. Chill. The lemon curd will continue to thicken as it cools. This recipe makes about 3 cups lemon curd. Store refrigerated for one to two weeks. After the lemon curd is thoroughly chilled, you can fold in up to a cup of heavy clotted or whipped cream to lighten the texture and flavor, if you wish.
Lemon curd, by the way, is simply the best-known in a family of fruit curds. Other popular fruit curds include lime, orange, raspberry, cranberry, blackberry, and even mango. (Shout-out to the steam juicers among us–fruit curd is another way to use your juice!) To make fruit curd using any other juice, just replace the cup of lemon juice with a cup of other juice and continue with the recipe. If using lime or orange, add the zest of that fruit. If using another fruit, either leave the zest out or add some lemon, lime, or orange zest. Imagine cranberry curd with orange zest! (Yum.)
Submitted by: suzanne-mcminn on July 15, 2010