1. Mix sugar and water or coffee in small heavy saucepan; cook over medium heat until sugar is completely dissolved, 2 to 3 minutes.
2. Gradually stir in cocoa powder and salt until completely blended.
3. Bring to boiling over medium-high heat, stirring constantly.
4. Remove from heat. Add butter and vanilla and stir until butter is completely melted.
5. Spoon immediately over ice cream or another favorite dessert.