PREPARE and bake cake according to package directions, using two 9-inch round cake pans.
LINE an 8-inch round baking pan with plastic wrap.
COMBINE cream, sweetened condensed milk and vanilla in large bowl. Cover and chill 30 minutes. Beat 4 to 5 minutes with electric mixer or until soft peaks form. Cover and freeze 1 1/2 hours or until partially frozen. Gently fold in chips, if desired. Spoon into prepared pan. Cover and freeze at least 6 hours, or until firm.
PLACE 1 cake layer on serving platter. Invert ice cream onto cake layer; remove plastic wrap. Top with remaining cake layer.
STIR enough milk into frosting to reach slightly soft spreading consistency. Quickly spread frosting over entire cake. Loosely cover with plastic wrap. Freeze until serving time.
REMOVE from freezer 15 minutes before serving. Decorate as desired with purchased frostings.