Stir together first 4 ingredients in a large heavy saucepan over medium heat, and cook, stirring constantly, 3 to 4 minutes or until sugar dissolves and butter melts.
Whisk about one-fourth of hot sugar mixture gradually into eggs; add egg mixture to remaining hot sugar mixture, whisking constantly. Cook over medium-low heat, whisking constantly, 15 minutes or until mixture thickens and coats back of a spoon.
Remove from heat; cool completely (about 1 hour), stirring occasionally. Store in an airtight container in refrigerator up to 2 weeks.
