Start the water boiling. Mix cocoa, sugar, and salt in a small saucepan. Stir in enough boiling water to form a smooth paste, then stir in the remaining water.

Bring to a simmer over medium to medium-low heat, stirring often. Let simmer 2-3 minutes (I like the longer time as it makes the syrup a bit thicker), stirring occasionally.

Remove from heat and add the vanilla. Let cool.

Scrape it into a bottle or jar and store in the refrigerator 3-4 weeks (or longer?).