In a 2 quart saucepan, combine sugar, cocoa and milk, stirring with a whisk until completely combined and smooth.
Turn heat on stove to medium and stirring occasionally, bring to a boil.
Stir in butter and continue to boil until sauce thickens, about 5 minutes.
Remove from heat, stir in vanilla and salt. Let cool for 5 minutes before serving over ice cream. Store in container in the refrigerator. To reheat, microwave for 30 seconds at a time until warm.
