In a small saucepan, combine 3/4 cup sugar and water. Bring to a simmer, stirring until sugar dissolves. Remove from heat. Add the chocolate. Whisk until the mixture is completely melted and smooth. Whisk in the rum. Pour into a large bowl and set aside. In a medium mixing bowl, using an electric mixer set on high speed, beat egg whites until soft peaks form. Gradually beat in remaining sugar until stiff, but not dry, peaks form. Whisk one quarter of beaten egg whites into chocolate mixture. Fold in remaining whites, then the whipped topping. Pour mousse into dessert dishes. Cover with plastic wrap and chill for at least 4 hours. Garnish with additional whipped topping, berries and walnuts.