I need to ask a couple of background questions before I can answer.

Did you say that the little girl is just a year old? If so, she's not ready to read or interpret or even listen to/understand a letter, as you know, and sending one would mean that someone else would have to keep it and save it for her. If you trust that her adoptive parents would reliably do that, then fine. But if not, one of the things you might do is keep a sort of journal for her and put down what you want to say now and in the future, then give it to her later when she's more grown up and ready to handle what you're saying. Right now, any letter you write is going to be more for yourself than for a one-year-old.

Do you know if the plan is that, once she's old enough, she'll be told the truth about being adopted and that you're her real biological father? When the time comes for you to communicate, is there any legal restriction that would prohibit your contacting her? That is, was it meant to be a closed adoption? Those are things to think about.

That was a very generous and good decision you two made, Frickr, to let her be raised by people who are better able to be parents. I wish more people would do what's best for babies like that.