In a large, heavy pot whisk together the egg, yolks, sugar, and lemon juice until smooth.
Place the pot over medium heat and whisk very frequently until the mixture is warm. Continue whisking until the mixture thickens and turns buttery yellow, about 7 to 10 minutes. Adjust the heat as needed to make sure the mixture does not simmer, but stays hot. When the mixture coats the back of a mixing spoon it is done. You can also use a candy thermometer to measure the temperature, which should be between 170° and 175°F.
Remove the pot from the heat and whisk in the butter one tablespoon at a time, until each addition of butter is melted and incorporated. Place a piece of plastic wrap over the top (this prevents a skin from forming) and allow to cool. Refrigerate for up to 3 days.
Notes
Tips on Making Lemon Curd
Just keep whisking. Don’t get distracted and wander far away from the stove.
Add the butter one piece at a time and whisk until it is incorporated before adding the next. This will help it emulsify and stay thick and smooth.
Remember the necessary chilling time—it definitely needs that time to firm up—and do press the plastic wrap onto the surface which will prevent a skin from forming.