Melt chocolate and butter in a large, heavy saucepan over low heat, stirring constantly. Add sugar, stirring until dissolved. Add milk, and cook, stirring constantly, 2 to 3 minutes or until thoroughly heated. (Do not boil.) Remove from heat. Stir in vanilla extract and salt. Cover and chill leftover sauce up to 2 weeks.
To reheat: Microwave sauce in a glass bowl at HIGH 1 minute. Stir and microwave at HIGH 1 more minute or until sauce is thoroughly heated.