In a small saucepan, whisk together the cocoa powder, sugar and salt until there are no lumps. Mix in the water.
Heat the saucepan on the stove over medium high heat and bring to a boil.
Reduce to a simmer, stirring constantly. Allow to simmer for around 1-2 minutes, then remove from heat. Allow to cool completely, then mix in the vanilla.
Pour the chocolate sauce in a mason jar, and store in the fridge.