ERBB2 and glycogen storage disease VI: Estimated Enrollment: 12- Histologically confirmed HER2-positive metastatic breast cancer- should not have received > 2 lines of chemotherapy for metastatic disease- Measurable disease in the brain, defined as at least 1 lesion measuring >= 5 mm on imaging at the time of registration- Any uncontrolled neurological symptom attributed to CNS metastasis or leptomeningeal disease or Previous treatment with Palbociclib is not allowed