Line the bottom and sides of a 9-inch springform pan with ladyfingers, placing cookies close together.
Cook sugar, 1/2 cup whipping cream, and butter in a heavy saucepan over medium heat, stirring constantly, 5 to 7 minutes or until sugar dissolves. Add chocolate, vanilla, and, if desired, liqueur, stirring until chocolate melts. Remove from heat; cool slightly.
Fold whipped cream into chocolate mixture. Pour one-third mixture into prepared pan; top with half of remaining ladyfingers. Repeat layers, ending with chocolate mixture. Chill at least 4 hours. (Cover and freeze up to 1 month, if desired. Thaw in refrigerator 8 hours.) Carefully remove sides of springform pan. Garnish, if desired.
Note: We used Ghirardelli semisweet chocolate squares.