Step 1Place the zest and sugar in a food processor fitted with a metal blade. Process until the zest is minced, about 20 seconds.
Step 2Place the sugar mixture, lemon juice, eggs, egg yolks and butter in a heavy-bottomed, non-aluminum saucepan. Use a whisk to mix until smooth. Cook over medium-low heat, using a wooden spoon to stir constantly until the mixture is thick enough to coat a spoon but does not come to a boil, about 8 to 9 minutes.
Step 3Immediately transfer the curd to a fine-mesh strainer. Strain, place plastic wrap on the surface of the curd, cool completely and refrigerate, covered airtight. (This can be kept up to 3 weeks, refrigerated. If curd becomes too thick, stir until flowing.)