Outline what is most impressive about this employee and why you like working with him/her. Address the letter to the person's superior but send the person being praised a copy as well.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I wanted to take the time to let you know how highly we think of <name>. <He/She> has consistently gone out of <his/her> way to <services performed>.
<Optional additional details.>
I'm sure that you value <employee first name> as much as we do, but I had to put it in writing. I wish all my business interactions were as pleasant and professional as working with <employee first name>. <He/She> is definitely one of your company's greatest assets.
encl: <List of enclosed items goes here>