Place the uncooked rice in the bowl of a blender and grind them briefly to break the rice grains into pieces. Add the milk and water, whole cinnamon stick, and vanilla bean. Cover tightly, refrigerate, and allow to soak in the refrigerator until the next day.

Remove the cinnamon stick and vanilla bean and discard.

Add the agave to the milk mixture and blend at the highest speed for about a minute, perhaps less if you have a high-powered blender. Allow the solids to settle briefly. Pour the horchata through a tea strainer or nut bag into serving glasses full of ice. Sprinkle with ground cinnamon and serve immediately.