In a 3-quart saucepan, whisk together the cocoa, sugar and salt until no lumps of cocoa remain. Whisk in the cold water (it will resist combining at first). Turn on the heat to medium to medium high and bring to a boil- whisking pretty consistently.
Once it comes to a boil, turn down the heat so that it is still boiling, but not in danger of boiling over- medium low worked well for me. Continue cooking and stirring for 5 to 7 minutes, or until the mixture has appeared to thicken slightly. Be careful not to overcook, this is a fairly thin syrup.
Remove from heat and add the vanilla. Cool, pour into a glass jar and store in the fridge.
Makes about 1 3/4 cups of chocolate syrup and will keep for several weeks refrigerated.