Grind the basmati rice into a fine powder using a coffee grinder.

Place the ground rice, almonds, and cinnamon sticks into a large bowl with 3 1/2 cups of water.

Cut the vanilla beans in half lengthwise, scrape the seeds into the bowl, and then add the beans.

Cover the bowl and let it sit overnight (approximately 8-12 hours).

The next day, add the sugar and 1 1/2 cups of water to the mixture.

Puree the mixture in a blender until smooth.

Strain the mixture using a fine-mesh strainer or a Thai tea sock.

Serve the horchata chilled.