How to Make It
Beat butter and sugar with an electric mixer on medium speed until blended, about 45 seconds. Add eggs and egg yolks, 1 at a time, beating just until blended after each addition. Gradually add lemon juice to butter mixture, beating at low speed just until blended. Stir in zest. (Mixture will look curdled.)
Transfer mixture to a heavy 4-quart saucepan, and cook, whisking constantly, over medium-low until mixture thickens and coats the back of a spoon, 14 to 16 minutes.
Transfer curd to a bowl, and place plastic wrap directly on warm curd (to prevent a film from forming). Chill until firm, about 4 hours. Refrigerate in an airtight container up to 2 weeks.