How should a student address a professor in a letter of recommendation? I am a graduate student in Computer Science, and I recently received an email from my department requesting that I write a letter of recommendation for a professor who is being considered for a promotion. The letter requests that I address the professor's teaching abilities, as well as my experiences in regards to conducting research with the professor. It is also likely that the professor specifically listed me as a person to write a letter of recommendation.

I have a very positive relationship with this professor, and plan on writing an absolutely stellar letter on his/her behalf. However, I am unsure of how to address the professor in the letter.

This professor strongly prefers that students address him/her by his/her first name. This is part of what makes this professor so great: s/he is able to come down to a student's level, and is excellent at communicating complex topics because of it. With that in mind, I would prefer to use his/her first name in my letter as I think it communicates part of the "charm" that I want to convey in regards to his/her teaching ability. On the other hand, I don't want to come across as disrespectful to the committee who will be reading my letter, nor do I want the committee to take my letter less seriously.

My question is: can and should I address the professor by his/her first name when writing this letter of recommendation, or should I address him/her as "Professor Xyz"? It may be worth noting that most professors in my department choose to be addressed by students on a first name basis, though in their request for a letter, the professor was referred to by their proper title (Professor Xyz).
I wouldn't worry a lot about it.  The worst that can happen is that part of the letter sounds a little off, but I'm sure promotion committees see much worse in letters from students, and understand it doesn't reflect on the professionalism of the professor.

If I had to chose, though, my personal inclination would be to write "Professor X" in the letter, rather than the professor's first name.  It's much easier to go wrong in the direction of being too informal, rather than being too formal.