Add the first 4 ingredients to a large bowl and stir.
Add the last 3 ingredients to a small bowl or cup and stir. Add the wet to the dry and beat with a hand mixer, starting on low and then increasing to high, until it's very smooth and there are no lumps remaining from the cocoa or sugar, a couple of minutes. Scrape the sides as needed.
You can use it right away if needed, but I recommend making it the day before you need it. It will become slightly sweeter, thicker and the sugar dissolves perfectly overnight. Store it at room temperature. If you store it in the fridge, it will become a lot thicker.
This makes plenty to glaze an 8 or 9 inch round single layer cake or an 8X8 square cake, brownies, bars or whatever you like! You can also use it on cupcakes by smoothing it out with the back of a spoon. It should be enough for a dozen.
Note: It is not recommended to heat this glaze. The texture will dramatically change, as tapioca becomes eggy when heated. Store it at room temperature.