The purpose of this study is to test the safety of a drug called BYL719 at different dose levels. The investigators want to find out what effects, good and/or bad, BYL719 has on the patient and breast cancer. BYL719 will be given with either letrozole or exemestane to patients with HR+ locally-advanced or metastatic breast cancer. When the recommended phase II dose of BYL719 in combination with letrozole or exemestane has been determined in the dose-finding phase, an additional 10 patients will be enrolled onto each arm in an expansion phase of the study. The purpose of the expansion phase is to further define the safety and feasibility of BYL719 in combination with letrozole or exemestane at the recommended phase II dose, and to estimate efficacy.