Set out a medium heavy-bottomed saucepan, a whisk, a heatproof spatula, and a fine-mesh strainer. Place a clean glass jar or bowl nearby for the finished lemon curd.
Off the heat, add the egg yolks, whole egg, and granulated sugar to the saucepan. Whisk until the mixture is smooth, slightly thickened, and lightened in color, about 1–2 minutes.
4 large egg yolks, 1 large egg, 3/4 cup granulated sugar
Whisk in the fresh lemon juice, lemon zest, and a pinch of fine sea salt until fully combined. The mixture will be thin at this stage.
1/2 cup fresh lemon juice, 1 tablespoon lemon zest, 1 pinch fine sea salt
Place the saucepan over medium-low heat. Cook the mixture, whisking constantly, for 8–12 minutes. Maintain gentle, steady heat; you should see light steam and small bubbles around the edges but no vigorous boiling.
Cook until the lemon curd is thick enough to coat the back of a spoon and a finger swiped through leaves a clean line. It should look glossy and have the texture of loose pudding; it will thicken more as it cools.
Remove the pan from the heat. Add the cold butter cubes a few at a time, whisking or stirring with the spatula until each addition is fully melted and incorporated, creating a smooth, creamy curd.
6 tablespoons unsalted butter
Pour the hot lemon curd through a fine-mesh strainer into the clean bowl or jar, using the spatula to press it through. Discard any solids left in the strainer.
Press a piece of plastic wrap or parchment directly onto the surface of the lemon curd to prevent a skin from forming. Let it cool at room temperature for about 30 minutes, then refrigerate for at least 2 hours, or until fully chilled and thickened.
plastic wrap or parchment paper
Once chilled, remove the cover and give the lemon curd a quick stir. Use as a spread, topping, or filling for cakes, tarts, bars, pancakes, waffles, yogurt, or ice cream.