Whisk together the juice, zest, sugar, and eggs in a 2-quart heavy saucepan. Stir in the butter and cook over moderately-low heat, whisking frequently, until the curd is thick enough to coat a spoon, about 6 minutes. Strain the curd through a mesh strainer or sieve to remove any cooked egg bits.
Transfer the curd to a bowl, cover its surface with plastic wrap to prevent a skin from forming and chill at least 1 hour, until cold.
Try making curd with any citrus fruit, like limes, oranges, grapefruit, tangerines, etc. You can also make curd with purees of berries such as raspberries or blackberries.
Recipe courtesy of Kelsey Nixon