Add the water or coffee, sugar, and cocoa powder to a small saucepan. Whisk until the cocoa powder is completely combined with the water.
Heat the chocolate mixture to a simmer, whisking occasionally.
Once simmering, remove from heat and stir in the vanilla extract. Allow to cool for about 10 minutes, then transfer to a jar or other airtight container and store in the fridge for up to 1 week.