I too agree that it’s great you’re asking! You offered the candidate who stutters a little extra time, which is a reasonable accommodation. So your company is ADA compliant. If the person volunteers that they stutter, you might ask them how they would like you to respond or react. That goes such a long way towards making the stutterer comfortable so they are not sitting and thinking, “OMG, hows she going to react when I stutter?” Once the elephant in the room is acknowledged, it’s easier to proceed with what’s at hand.