In a medium saucepan, bring condensed milk and brown sugar to a boil over medium-high heat, whisking constantly. Reduce heat, and simmer (low) for 5 minutes or until mixture thickens. Continue to whisk constantly.
Remove pan from heat and immediately whisk in butter, salt, and pure vanilla extract*.
Notes
*This caramel sauce will harden. Use right away or put the caramel in the refrigerator. Caramel will be good for 5-7 days.To re-use: Simply put the caramel in the microwave at 50% power for about 30 seconds to 1 minute to allow the caramel to get smooth again.