Special equipment: a 3-quart (9-by-13-inch) glass or metal baking pan
Put 6 or 7 glasses in the refrigerator to chill.
Cook 3 cups water, the mint and sugar in a medium saucepan over medium-high heat, stirring occasionally, until the sugar is dissolved, about 2 minutes. Strain through a fine-mesh strainer into a medium bowl, pressing on the mint with the back of a spoon to extract all the juices. Let cool completely, or chill over an ice bath.
Stir in the bourbon, and pour into a 3-quart (9-by-13-inch) glass or metal baking pan. Cover with plastic wrap, and freeze overnight.
Scrape the surface of the ice with a fork in long, light strokes to create a snowy texture. Break up any remaining lumps. Freeze the granita again for at least 2 hours before serving. Fifteen minutes before you are ready to serve, put 6 to 7 glasses in the refrigerator to chill. Serve the granita in the glasses, garnished with fresh mint leaves.
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