Melt chocolate and butter in a large, heavy saucepan over low heat, stirring constantly. Add sugar and espresso, and cook, stirring constantly, 30 seconds or until blended. Add milk, and cook, stirring constantly, 3 minutes or until thoroughly heated and sugar is dissolved. (Do not boil.) Remove from heat. Stir in vanilla extract and salt. Cover and chill sauce up to 2 weeks.
Note: To reheat, microwave sauce in a microwave-safe bowl, stirring occasionally, at HIGH for 15- to 30-second intervals or until warm.