How to Make It
In a large bowl, stir the coffee with the sugar until the sugar is dissolved. Pour the coffee into two 9-by-13-inch glass or metal baking dishes and freeze until ice crystals begin to form around the edges, about 2 hours. Using a fork, break up the ice crystals and stir them into the middle. Continue freezing, stirring and scraping every 30 minutes, until the granita is thoroughly frozen and the ice crystals are fluffy, about 2 hours longer.
In a large bowl, mix 1 cup of the coffee with the gelatin and let stand for 1 minute. In a saucepan, bring the remaining 3 cups of coffee to a boil over moderate heat. Add the hot coffee and the sugar to the bowl and stir until the gelatin and sugar are completely dissolved. Pour the mixture onto a 11-by-17-inch rimmed baking sheet and let stand at room temperature until just beginning to set, about 15 minutes. Carefully transfer the baking sheet to the refrigerator and chill the jello until firm, about 3 hours.
Using a ruler and a sharp knife, cut the jello into small cubes. Gently loosen the cubes from the baking sheet with a spatula. Cover and refrigerate.
In a small saucepan, warm the condensed milk with the cinnamon. Pour the warm milk into a small pitcher.
Spoon a generous 1/2 cup of granita into glasses or ramekins. (If you have room in your freezer, you can do this a few hours ahead.) Top the granitas with 1/4 cup of jello squares and 2 tablespoons of the cinnamon milk and serve at once.