Halve the avocado, remove the pit, and scoop out the flesh from one half. Rinse the spinach under cold running water and drain it well so no excess moisture remains.
Pour the water into the blender first. This prevents the powder from sticking to the bottom or blades. Then add the drained spinach, the avocado flesh, and a scoop of vanilla protein powder.
Turn the blender to a low speed and gradually increase to maximum. Let the machine run continuously for over a minute until all ingredients are completely blended into a smooth liquid with no lumps or leaf pieces remaining.
Pour the smooth mixture directly into a large, sturdy glass. Drink the smoothie right away before the heavier ingredients settle to the bottom.