This study has public funding from the National Cancer Institute (NCI), part of the National Institutes of Health (NIH) in the United States Department of Health and Human Services. We do research studies to try to answer questions about how to prevent, diagnose, and treat diseases like cancer. This research study will consider those who have a specific type of uterine cancer called endometrial serous carcinoma or carcinosarcoma. The tumor has been tested and has been found to be human epidermal growth factor receptor 2 (HER2) positive. This means that the tumor contains HER2, which is a protein that can promote the growth of cancer cells. There are drugs that specifically act against HER2, and we are studying whether these drugs work in women with HER2 positive uterine cancer. This study is being done to answer the following question: Can we lower the chance of your endometrial cancer coming back and causing death by adding a drug or drugs that target HER2 proteins in addition to the usual combination of chemotherapy drugs? We are doing this study because we want to find out if this approach is better or worse than the usual approach for your endometrial cancer.