Combine sugar, cocoa, and cream in 2-quart saucepan.
Stir with whisk until smooth.
Cook over medium heat, stirring occasionally, until sauce comes to a boil.
Stir in butter.
Boil until sauce thickens, stirring occasionally, approx. 5 minutes.
Remove from heat.
Stir in vanilla and salt.
Let cool five minutes.
Serve or store in container in refrigerator.