Ingredients
1/2 cup sugar
1/4 teaspoon kosher salt (we use Diamond Crystal)
1 large egg, plus 3 large yolks
1 1/2 teaspoons finely grated lemon zest, plus 1/3 cup fresh juice (from 2 lemons)
3 to 4 tablespoons unsalted butter
Preparation
In a nonreactive pot, such as enameled or stainless steel, whisk together sugar and salt. Whisk in whole egg and yolks, then immediately whisk in lemon zest and juice.
Place pot over medium heat and bring mixture to a simmer, whisking constantly, until bubbling in center and thickened, about 5 minutes. Remove from heat and whisk in butter. (For a thicker consistency, use 4 tablespoons; for thinner—say, for use as a sundae topping—use 3 tablespoons.)
Strain through a fine-mesh sieve set over a bowl, to remove any zest and cooked pieces of egg white. Press a piece of plastic wrap directly on surface and let cool completely, about 20 minutes. Refrigerate until thickened and cold, at least 1 hour or, in an airtight container, up to 1 week (or freeze in an airtight container up to 2 months).
You can also swap in other citruses: For orange curd, replace half the lemon juice with fresh OJ and all the zest with orange zest. For lime curd, replace all the juice and zest with lime juice and zest.
