Rinse the rice under cold water until the water runs clear

In a bowl, combine rinsed rice with cinnamon stick and 480 ml cold water; soak for at least 4 hours or overnight for best flavor

After soaking, transfer rice and cinnamon water (discarding rice if preferred, or blending it) into a blender; blend until rice is broken down and water is milky

Strain the mixture through a fine-mesh sieve or cheesecloth into a large pitcher to remove solids

Add remaining 720 ml cold water, milk, granulated sugar, and vanilla extract to the pitcher; stir until sugar dissolves

Chill the horchata in the refrigerator for at least 1 hour

Serve over ice and sprinkle a touch of ground cinnamon on top if desired