In medium saucepan, combine semisweet chocolate and the 1/4 cup whipping cream. Cook and stir over low heat until smooth. Gradually stir in coffee and brown sugar; heat through. Meanwhile, combine whiskey and creme de cacao. Pour 1 tablespoon of whiskey mixture into bottom of each of 6 mugs. Add coffee mixture to each. Top with whipped cream; sprinkle with grated chocolate, if desired. Makes 6 (about 5-ounce) servings.