In a large bowl combine the rice, almonds and cinnamon stick. Add the water. It should be hot water from a kettle that has been cooled down a bit.
Cover and refrigerate overnight. (I let it sit for an extra night- 36 hours, which I think helps in the grinding of the cinnamon stick) If you don't want to wait as long, just remove the cinnamon stick before blending.
Pour the mixture into a blender and add the sugar.
Blend on high for several minutes, or until the mixture is as smooth as it will possibly get (it will still have a hint of grittiness, this is fine)
Strain through a fine mesh sieve (or a sieve line with cheesecloth) You can avoid any grit or pulp in the final drink by not pressing as vigorously. I don't mind it. It will just mean shaking the bottle etc before using. Feel free to strain a second or even third time if you want.
Pour into a glass jar with a lid, and add the remaining milk. Taste to see if you want to add more sugar.
Serve over ice. Shake well before using. In Oaxaca they serve it with cubed cantaloupe and broken pecans.
If you find it too thick, feel free to thin it out with more milk or even water.
You could easily make this into a rum cocktail by adding 1 or 1 1/2 oz of rum of choice and ice to a tall glass before pouring in the horchata. Grate some extra cinnamon on top, or how about some flaked coconut?
You can also make a cocktail using 1 oz of tequila, 1/2 oz of triple sec, and a splash or more of orange juice. In a tall glass over ice. Just add an umbrella!
Adapted from 'Mexico One Plate at a Time' by Rick Bayless.