Cut pears in half vertically. Gently cut or scrape out core. Dip cut edges in lemon juice. Sprinkle about ½ tablespoon sugar over the cut edges of each pear half.
Place each pear, cut side down, in a large, heavy skillet over medium heat. Sprinkle 1 tablespoon sugar in skillet around pears. Cook 7 to 9 minutes or until sugar is melted and golden and cut surface of pears is golden. Remove pears from skillet.
Meanwhile, heat half-and-half in a 2-cup glass measuring cup in microwave oven on high (100 percent power) 45 to 60 seconds or until steaming hot but not boiling. (Watch carefully so it does not boil.) Immediately add chocolate to hot half-and-half. Allow to stand 3 to 5 minutes or until chocolate melts. Stir to blend and melt chocolate. (If necessary, return to microwave oven and heat on high power in 15 second intervals, stirring after each, until melted.) Stir in vanilla.
Place a small scoop of frozen yogurt in each dessert bowl. Place pear halves on yogurt. Drizzle any collected caramelized sugar over pears. Drizzle with dark chocolate sauce. Serve immediately.