A well-written thank you note/letter can make the difference between winning the job, or not. Your thank you letter will be prepared after your job interview and fully customised to the role and the interviewer(s). Its purpose is to communicate information that was not shared during the interview, respond to any concerns that may have been raised, reinforce points from the interview, distinguish you as different from other candidates, and of course build a relationship with the hiring authority and keep you top-of-mind.
There is no reason, particularly with a thank you letter, to write a formal letter on letterhead. Instead make it easy for the reader by delivering your letter via email message. It’s quick and efficient and can easily be saved in email or in the database of a company or recruiter.