In a medium sized sauce pan over low heat, whisk together the sugar and eggs until smooth and combined well. Whisk in lemon juice, lemon zest and  butter.

Cook over moderately low heat, whisking frequently, until curd is thick enough to hold marks of whisk and first bubble appears on surface, about 6 minutes.

Push mixture through a fine, mesh sieve and pour into clean mason jar or container.

Once the curd has cooled to room temperature, cover it with a lid and store it in the refrigerator for about 2 weeks. The curd will thicken as it cools.