Combine the rice and 2 1/2 cups steaming hot water in a large bowl. Soak at room temperature for 2 hours, the rice will not be completely soft, this is OK. Alternately, you can use room temp water and soak for 8 hours or overnight. (see notes for additional methods).Pour the rice and water into a blender. Blend until creamy, 2 to 3 minutes. Add the honey or maple syrup, vanilla, nutmeg, cinnamon, and a pinch of salt. Blend until combined. Pour the mixture into a large pitcher or jar. Add the milk and stir to combine. Chill until ready to serve. Stir well before serving.To serve, rim each glass with cinnamon sugar, if desired. Fill glasses with ice. Add the horchata, then pour over the coffee, using an amount to your liking. Add the Kahlua, if using, and stir to combine. Sprinkle with cinnamon. Enjoy!