Whisk together the granulated sugar, eggs, and lemon juice in a stainless steel or heat proof bowl. Place over the top of a pan of simmering water and cook, whisking constantly so the egg doesn't scramble, until the mixture thickens to the consistency of sour cream, about 10 minutes. Keep a close eye on the curd…if you start to see bits of egg and it's not thick enough yet, reduce the heat. Don't worry, any bits of egg will be strained out.

Remove from heat and pour the curd through a fine strainer placed over a small bowl.

Add the butter, and stir until completely melted, then stir in the lemon zest. The curd will thicken as it cools. Many lemon curd recipes include more butter, so if you prefer more richness, add more butter as desired.

Once the curd has cooled, place plastic wrap over the surface of the curd to keep a skin from forming, and store in the refrigerator.