Sift cocoa into medium saucepan. Add brown sugar and mix well.
Add boiling water and vanilla, if using, and stir with wooden spoon until smooth.
Bring to the boil, while stirring all the time and then simmer gently for about 10 minutes until it thickens. The sauce will thicken more on cooling, so don't simmer any longer or it may burn.
Cool sauce with lid on saucepan until cooled enough to pour into a container and store in fridge. I use a sauce bottle with squirty top, such as the ones you get from a supermarket which you can use for tomato sauce.