In a high-speed blender, combine the water, dates, avocado, cacao powder, vanilla, and spinach, if using, and blend until very smooth. Taste the pudding-like mixture to make sure there's enough sweetness and chocolate flavor to your liking, and adjust anything to your taste. (Keep in mind that the flavor will be diluted a bit more once you add the ice.)
Add the ice and blend again, until the smoothie has more of a milkshake-like texture. You can add as much ice as needed to achieve the texture you want, but keep in mind that extra ice will dilute the chocolate flavor. Serve right away.