Combine the water, sugar, and salt in a sauce pan. Bring the mixture to a boil, and hold it at a boil for 1 minutes. Drop the heat, and whisk in the cocoa powder. When the chocolate sauce is at a simmer, and the cocoa powder is fully combined, remove the mixture from the heat and stir in the vanilla. Allow your chocolate sauce to cool before storing in a canning jar or other airtight container placed in the fridge for up to 2 months.