Soak the brown rice and chopped macadamia nuts in a bowl with water for 12 hours. Drain the mixture after soaking.
Place the soaked rice, nuts, cinnamon, vanilla essence, and macadamia milk into a blender. Blend until the mixture is smooth and the rice and nuts are properly ground.
Strain the blended mixture through a muslin cloth or milk bag into a jug to remove any solids.
Pour the horchata into glasses over ice. Sprinkle extra cinnamon powder on top before serving.