In a small pot set over medium-high heat, mix water and sugar until dissolved. Bring to a simmer. Reduce heat to low and simmer for 3 minutes, stirring regularly.
Stir in cocoa powder and salt, and continue stirring and simmering for about two more minutes or until cocoa powder is all dissolved and mixture is smooth and slightly thickened. Remove from heat and stir in vanilla. Store chocolate syrup in a jar in the refrigerator.
This syrup will thicken as it cools and should have the consistency of store bought chocolate syrup once chilled. It is not supposed to get thick like a fudge sauce.
It will keep in the refrigerator for three to four weeks.