Stir the coffee, milk, sugar and chocolate until well combined. Pour the mixture into a 8-by-8-by-2-inch glass baking dish. Freeze the granita until firm, stirring every 45 minutes, about 4 hours. Cover the baking dish and keep frozen. Before serving, scrape the surface of the granita with a fork until crystals form. Using an ice cream scoop, scoop out the granita and serve.
Cook's Note: This granita can be prepared 3 days ahead.