Here's a general format that I've been following, that one of my undergrad advisors approved of So far, POI's responded positively and with some intel as to when decisions could be made.
Dr. [name],
I really enjoyed speaking with you [this week/during interview week/last month] and hearing your perspective regarding [some topic you discussed in interviews, research interest overlap], as well as learning more about your work regarding [other work they focus in, whether it overlaps with your work or is a potential future research path]. I see [school's name] program as a great fit for me and an opportunity to expand upon my current research interests and explore new interests as well!
Thank you for your time,
[sign off]