Simply replace the unfilled areas such as "" and "[date]" with your information.
It has been really great working for you this year. You have been tremendously supportive of me, and I am very glad to call you my boss. I hope that you have a very wonderful birthday, and an equally wonderful year to come.
Please let me know if there is anything I can do to help you out at work or make your job easier. I know you worry about our department and you always look out for all of us employees. For today, your birthday, take a little break and relax! We promise to hold up the building for you for a bit.
Best wishes for a great year,