Add the chufa nuts to a large bowl and fill with enough cold water to cover the nuts by one inch. Soak the chufa nuts for 24 hours.
After 24 hours, drain the chufa nuts and rinse them under cold water. Add half of the nuts and all of the cinnamon to a blender with ½ of the cold water. Blend on high until you have a smooth mixture. Add the remainder of the nuts, the remainder of the cold water, sugar and lemon zest and blend again until you have an even, smooth mixture.
Let it settle for a half an hour.
Press the paste through a fine mesh sieve into a large bowl, pushing down on the mixture to release all the liquid. Discard what is left in the sieve. Transfer the horchata to a pitcher and refrigerate for at least one hour to let the flavors develop.
Finish in the freezer for 20 minutes until slightly slushy. Remove from the freezer, give the horchata a stir, pour into glasses and serve with a straw.
Serve as a cool afternoon treat on a hot summer day!