Denver-based fast-casual chain Chipotle Mexican Grille Inc. said today that Brian Niccol, chief executive of Taco Bell, will be its next CEO, replacing founder Steve Ells. Niccol has headed Taco Bell — the Mexican-fast-food brand of Louisville, Kentucky-based Yum! Brands Inc. (NYSE: YUM) — since 2015. He will become just the second solo CEO at Chipotle (NYSE: CMG).