self-possessed

Effortlessly cool(on the surface anyway)and thoughtful, especially in the face of adversity or setbacks; a more ritzy way of describing "composure".

The new waiter Charlie's self-possessed response to the boss's tantrum and finger-pointing quickly established him as one the most powerful employees on the payroll: " The delivery list not having been updated in years is really no one's fault but your own. None of us here were hired to be mindreaders. Sir."