Place chocolate in a microwave-safe bowl. Cook on high for 1 to 2 minutes, stirring frequently, until mostly melted. Transfer chocolate to a heavy-bottomed saucepan over low heat. Stir in sugar, butter, and salt until combined.

Stir in cream, a little at a time, until smooth. Heat through, without boiling, then remove from heat and stir in vanilla. Store sauce in the refrigerator.