The purpose of this study is to evaluate if the study drug palbociclib has anti-tumor activity against the breast cancer that has spread to the brain and also to determine the overall radiographic response rate in the CNS. Palbociclib is an anti-cancer medication that has been shown to stop cancer cells from growing. It has been approved in hormone positive breast cancer, along with other hormone therapies and has been found to be effective. The preclinical studies suggest that the drug may also have activity in other types of breast cancer, such as HER2 positive breast cancer. The purpose of this study is to see if the study drug is effective in patients with brain metastasis, who have HER2-positive breast cancer.