In 2-quart saucepan, mix all filling ingredients except butter with whisk. Heat to simmer over medium heat, stirring constantly, until mixture thickens and coats back of wooden spoon, 4 to 5 minutes. Strain sauce through strainer into small bowl. Beat in butter with whisk. Press plastic wrap directly onto surface of lemon curd; refrigerate at least 3 hours but no longer than 1 week.