Combine the rice and cinnamon stick with 4 cups water in a blender; pulse to coarsely grind. Transfer to a large bowl and add another 4 cups water; soak at room temperature for 3 hours.
Puree the rice mixture in a blender in batches until smooth. Strain through cheesecloth or a fine sieve into a pitcher. Mix in the sugar; chill.
Stir the horchata well before serving. Pour into ice-filled glasses; garnish with cinnamon sticks and a dusting of ground cinnamon.
Photograph by Kana Okada
Recipe courtesy Aaron Sanchez for Food Network Magazine