<p>I agree that you should absolutely not buy a professor a gift for writing a letter of recommendation. I think the most is a card to express your gratitude but even that may seem a little overboard considering how expected it is for faculty to write letters of recommendation. I think the safest bet is just a nice email thank you. </p>
<p>Someone mentioned buying a gift if you would never see the professor again. I think that’s okay but I’m on the fence about the timing. I think the best time to give that professor a gift is at or just before graduation. Any other time, regardless if you have no intention of taking a course with them, may lead to misconstrued intentions. Also, you never know if that person will end up teaching you again for some unknown reason.</p>