Mix together coffee, sugar, and espresso powder. Stir until the sugar has melted. Allow to cool completely. Pour into freezer-safe 9x13 baking dish and place in the freezer. Scrape with the tines of a fork to break up the granita every 30 minutes until there is no longer any loose liquid left in the granita, about 3 hours. Then, fluff the granita with the fork and freeze for an additional 30 minutes.