Place eggs and sugar into a small pot, whisk to combine. Add lemon juice, zest, and butter. Cook over medium-low heat whisking constantly until mixture thickens and coats the back of a spoon.
Transfer to a glass bowl and lay plastic wrap directly on the surface to prevent a skin from forming. Chill for 2 hours to set. Makes 2 cups.
The curd will last for 1-2 weeks in the fridge or can be frozen for 3 months.