Combine the first 4 ingredients in a large saucepan. Gradually add skim milk, stirring with a wire whisk until blended. Add the chopped chocolate; bring to a boil over medium heat, and cook for 1 minute, stirring constantly. Add the margarine, and cook for 1 minute, stirring constantly. Remove from heat. Place pan in a large ice-filled bowl for 10 minutes or until the chocolate mixture cools to room temperature, stirring occasionally.
Remove pan from ice. Spoon 1/2 cup chocolate mixture into a small bowl, and gently fold in whipped topping. Cover and chill chocolate-whipped topping mixture. Set aside the remaining chocolate mixture.
Line a 9 x 5-inch loaf pan with heavy-duty plastic wrap, allowing wrap to extend over edge of pan. Split ladyfingers in half lengthwise. Arrange 9 ladyfinger halves lengthwise in 3 rows, rounded side down, in a single layer in bottom of pan. Brush 1 tablespoon Grand Marnier over ladyfingers in pan. Spread about 1/2 cup chocolate mixture over the ladyfinger layer. Repeat procedure 3 more times with the remaining ladyfingers, Grand Marnier, and chocolate mixture, ending with ladyfingers (1 ladyfinger half will be left over). Cover; chill for at least 8 hours.
Uncover and invert the pan onto a serving platter; remove the plastic wrap. Sprinkle with powdered sugar. Serve cake with chocolate-whipped topping mixture. Garnish each serving with an orange slice, if desired.