Genuinely be upfront with her, gently reassure her that it's no big deal & that you don't mind Just whatever you do, DO NOT tell her to "slow down" or "take your time." Also me personally, I would take offense if someone told me my stutter was "cute," I wouldn't want someone to view my stutter as a fetish or trait b/c it undermines who I really am if I'm just known as the person who stutters  You could eventually tell her why you find her condition endearing & ask her about real life situations. If she was willing to address her stutter to you, chances are she would be inclined to explain more about it People who stutter inherently have an added weight on their shoulders every day & we typically are eager on the inside when others are willing to ask about it