Split ladyfingers in half lengthwise. Line bottom and sides of a lightly greased 8-inch springform pan with ladyfingers, cut side in; set aside.
Combine marshmallows, milk, and chocolate morsels in a microwave-safe bowl; microwave at HIGH 3 minutes or until marshmallows and chocolate are melted, stirring occasionally. Stir in vanilla; cool completely. Fold whipped topping into cooled chocolate mixture; pour filling into prepared pan. Cover and freeze 8 hours. Carefully remove sides of pan. Garnish, if desired.