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.
Cook's Note: 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.