Wash the rice and cashews to remove any starch or debris on the exterior then add 2 cups of cold water and a toasted cinnamon stick. Toasting the cinnamon releases the flavourful oils in the stick and will enhance your finished horchatta. Soak for at least 3 hours, or preferably, overnight.
Dump the water, rice, cashews and cinnamon into a blender and blend for a few minutes until it is very smooth. Add the sugar and an additional 2 cups of water and continue to blend for another few minutes.
Pour the mixture through a double mesh sieve into a bowl with a spout. Then pour the mixture through a sieve with even smaller holes (I used one made for separating oil from water). You could also wash your double mesh sieve and line it with a quadruple layer of cheesecloth. Use a spoon to stir the solids around, which will help the liquid pass through.
Your horchata should now be free of any chalkiness. If you want to serve it right away, add 2 1/2 cups of crushed ice. If you're going to store it, add 1 1/2 cups of water and refrigerate. Dust with ground cinnamon when serving.