Remove cake from cake pan and turn upside down onto a serving plate. Remove foil. In small bowl, place remaining 2/3 cup chocolate chips. In 1-quart saucepan, heat remaining 1/3 cup whipping cream and 1 tablespoon butter over medium heat, stirring until butter is melted and mixture comes to a boil. Pour cream mixture over chocolate and stir smooth. Pour the melted chocolate over the cake; frost top and sides using a butter knife or offset spatula. If desired, decorate with melted milk chocolate.