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).