In a saucepan over medium heat, whisk together the cocoa powder, sugar, salt, and water. Continue to whisk until boiling.
Continue boiling until mixture thickens, about 3 minutes. The mix will be thin, but will thicken as it cools.
Remove from heat and stir in vanilla. Allow to cool.
Store in an airtight glass container or plastic squeeze bottle.
Syrup will keep for up to one month in the fridge.