1/4 cup finely grated, loosely packed lemon zest (from about 5 to 6 medium lemons)
1/3 cup freshly squeezed lemon juice (from about 3 to 4 lemons)
1/8 teaspoon fine salt
6 tablespoons unsalted butter (3/4 stick), cut into 6 pieces, at room temperature
Fill a medium saucepan with 1 to 2 inches of water and bring it to a simmer over high heat; reduce the heat to low and keep the water at a bare simmer.
Place all of the ingredients except the butter in a large heatproof bowl and whisk to combine. Set the bowl over, but not touching, the simmering water and whisk constantly until the yolks thicken and the mixture forms ribbons when the whisk is lifted from the bowl, about 7 to 10 minutes. (Check to make sure the water does not boil by periodically removing the bowl from the saucepan using a pot holder or dry towel. If the water boils, reduce the heat so the eggs do not curdle.)
Remove the bowl from the simmering water; whisk in the butter one piece at a time, waiting until each piece is completely melted and incorporated before adding another.
Set a fine-mesh strainer over a medium bowl. Strain the curd, pressing on the solids and scraping the extra curd clinging to the underside of the strainer into the bowl; discard the solids left in the strainer. Press a sheet of plastic wrap directly onto the surface of the curd to prevent a skin from forming. Refrigerate until set and chilled, at least 3 hours. (The curd can be made and refrigerated up to 5 days ahead.)