In a bowl, stir together the coffee, sugar, vanilla and salt until the sugar dissolves. Pour into an 8-inch square baking dish and freeze for 2 hours. Remove from the freezer and use a fork to scrape and stir any of the ice that has formed. Return to the freezer for another 2 hours and repeat. The granita is now ready to serve with a dollop of lightly sweetened cream or drowned in a 1/2 ounce shot of limoncello.