In a small saucepan, combine condensed milk, brown sugar, and salt. Bring to a boil over medium heat, whisking constantly. Reduce heat, and simmer until mixture thickens slightly, whisking constantly. Remove from heat, and add butter and vanilla extract, whisking to combine. Use immediately.
Recipe by TeaTime Magazine at https://teatimemagazine.com/salted-caramel-pound-cake-recipe/