Mix sugar, cocoa and water. Bring to a boil and then simmer for 15 minutes.
Cool and add vanilla. Good in milk, over cake or on ice cream.
Notes
In my experience, simmering for 15 minutes was a bit too long and when cooled, the sauce was too thick. Remember, when you are reducing a sauce, that it will thicken considerably when cooled. If you do end up with too thick a sauce, you can thin it out with a bit of milk or half and half.