1. Whisk together eggs and yolks, then combine with sugar and lemon juice in a small heavy-bottomed saucepan. Cook over low heat, stirring constantly, until mixture coats back of a spoon, 12 to 14 minutes. Strain into a small bowl. Add butter a little at a time, stirring until smooth. Stir in lemon zest. Press parchment or plastic wrap directly on surface of curd to prevent skin from forming. Let cool completely in refrigerator before using, at least 2 hours and up to 2 days.