*I have made this both with Hershey's cocoa powder and Ghirardelli cocoa powder - both come out great but I recommend ghirardelli.
** express powder is totally optional, i have made it without but if you have it - it deepens the flavor of the chocolate
In a medium saucepan, combine the sugar and cocoa powder and whisk briefly to break up any clumps. Stir in the water and salt and bring to a boil over medium-high heat, stirring frequently. Once the mixture reaches a boil, reduce the heat to medium-low and let simmer until slightly thickened, about 5 minutes. Remove from the heat and let stand for 5 minutes. Stir in the vanilla. Let cool before you refrigerate. I store mine in a mason jar.