Muddle fig, mint leaves, and brown sugar cubes against sides of a cocktail shaker. (Fig needs to be fairly broken down.) Add bourbon and enough ice cubes to fill shaker (about 1 cup). Cover with lid, and shake vigorously until thoroughly chilled (about 30 seconds). Strain into a 10-oz. glass filled with ice cubes. Top with chilled ginger ale. Stir gently, and garnish, if desired. Serve immediately.