Boil some water. While it's boiling, add the sugar and cacao (or cocoa) powder to a mason jar (pint size minimum) and stir well until no white sugar is visible.
Slowly pour about 1/8 cup boiling water into the jar. Use a silicone spatula to begin stirring the water down into the powder mixture. Add a bit more water as needed and continue to stir. Keep stirring until the sugar is dissolved and there the syrup is smooth and shiny. It's OK if you don't use a full 1/4 cup. The amount of water used will determine the consistency of the final product. Less water yields a thick, fudgy sauce (great on ice cream).
You can kick up the flavor by adding cinnamon or cayenne to the mixture. You can also adjust the sugar content to your liking using a bit more or less sugar.
Keep refrigerated and use to taste with cold milk, coffee, hot milk, smoothies, etc.
I like to keep a jar of this on hand for when I need a cup of hot cocoa on a cold day or to have a glass of chocolate milk with my grilled cheese sandwich (or mac and cheese) or to drizzle over ice cream.