Cut chicken breasts into 1-inch chunks. In bowl, stir together chicken, cornstarch, soy sauce and pepper; let stand for 15 minutes.
Meanwhile, peel and cut mango into ½-inch chunks. Set aside.
In shallow Dutch oven or wok, heat oil over medium-high heat; stir-fry shallots until golden, about 3 minutes. Add chicken mixture, hot pepper and garlic; stir-fry for 4 minutes. Stir in mango and red pepper until combined.
Add chicken stock, fish sauce, lime juice and sugar; bring to boil and boil, stirring, until slightly thickened, about 2 minutes. Stir in mint.