Ingredients

• 7 to 8 large egg yolks (about 130 grams w/o shells), depending on size
• 1¼ cups + 2 tablespoons (275 grams) granulated sugar
• 4½ fluid ounces (about 133 ml) freshly squeezed lemon juice (4 large lemons)
• Pinch of kosher or sea salt
• 1 tablespoon (6 grams) finely grated lemon zest
• ½ cup (1 stick/113 grams) chilled unsalted butter, cut into pats

Directions

Add an inch or so of water to a medium, heavy nonreactive (noncorrodible) saucepan or bottom pan of a double boiler set. Bring water to a simmer over medium-high heat. Meanwhile, in a medium sized metal bowl (or separate glass bowl) or top pan (insert) of a double boiler set, beat yolks and sugar vigorously with a whisk until smooth and well blended, about 1 minute. (Mixture will be very thick at first, just keep at it.) Add lemon juice and salt, whisk until smooth. If mixed in separate glass bowl, scrape and pour into top pan of double boiler set.
Once the water reaches a simmer, reduce heat to low and place bowl over saucepan or top pan (insert) into bottom of double boiler. Do not allow water to touch the bottom of the metal bowl or top pan (insert) of the double boiler as this could scorch and possibly curdle the mixture. Cook whisking constantly until thickened, about 20 to 22 minutes. The mixture will change from translucent to an opaque light yellow color and will coat the back of a wooden spoon yet still be liquid enough to pour. Do not allow the mixture to boil or it will curdle.
Remove promptly from heat and immediately whisk in lemon zest to release oils. Add butter gradually, one piece at a time, whisking well to combine. Allow each addition of the butter to melt completely before adding more. If straining (this is optional to strain lemon zest and any coagulated egg) for a smooth curd, strain at once into a medium bowl and press strainer with the back of a spoon or rubber spatula until only the coarse residue remains. Discard residue.
Allow curd to cool; cover by placing a layer of plastic food wrap directly on top of the surface of the curd. The curd will continue to thicken further upon resting and chilling. Transfer to airtight container and refrigerate.