This is a "thank you letter" for a gift. So perhaps your boss have you a gift or someone else in your company gave you a gift. And here is how you would think. It's always good to send a thank you note whether it's professional or personal, thank you for a gift is much appreciated. It shows that you are a respectful and appreciative employee.
Subject Line: Thank You
Dear [Title and Name of Recipient],
I would like to extend my gratitude for the [gift] given to me by [Name] [yesterday/ last week/ last month]. I am sincerely thankful for the gift, and I will not forget it. It says a lot about the kind of company this is, and reminds me of how grateful I am to be a part of this team.
Warm regards
[Full Name]