Add all ingredients to a pint-size mason jar. Tighten the lid and shake vigorously for about 30 seconds.
Let sit for a few seconds so the bubbles disperse. Serve. If there seem to be small bits of undissolved cocoa powder, you can pour it through a fine-mesh strainer (the small kind meant for tea works well if you have one).
Notes
You can store in an airtight container for up to 24 hours in the fridge. Shake or stir before serving.
Use honey or agave instead of maple syrup.
You can try using less sweetener. I find that this amount works best to balance the cocoa powder, but you may feel differently, so taste and adjust.
You can also make this in a blender if you prefer.