- In a saucepan, off the heat, combine quickly the sugar, eggs, egg yolks, and lemon zest with a whisk until smooth. Stir in the lemon juice.
- Cook over medium heat, stirring with a whisk, until the mixture thickens or until a candy thermometer reads 76 °C (170 °F), about 4 to 5 minutes. Remove from the heat and strain immediately.
- Add the butter and stir until melted. Cover with plastic wrap directly on the curd. Let cool and refrigerate for about 3 hours or until completely chilled.
For a thicker curd, stir in an additional 30 to 60 ml (2 tablespoons to ¼ cup) of butter.