In a small heavy saucepan over medium heat, whisk together the eggs, sugar and lemon juice until all three are well blended. Be sure the heat isn't too high, or else the eggs might start to scramble and separate.
Add the butter and lemon zest, and cook until the mixture is thick and coats the back of a metal spoon. Whisk constantly during this time.
Transfer the lemon curd to a small bowl (preferably glass), and allow it to cool for 10 minutes. Once cooled, refrigerate it, covered, until cold.
