In blender, place all ingredients except mint sprigs. Cover; blend on medium-high speed about 30 seconds or until smooth. Pour into 8-inch square (2-quart) glass baking dish.
Freeze 1 hour. When ice crystals begin to form at edges of dish, stir mixture with fork. Freeze 3 hours longer, stirring every 30 minutes with fork, until firm. Serve frozen or let stand at room temperature 5 to 15 minutes before serving. Scoop into individual dessert bowls. Garnish with mint sprigs.