birthday dinner in Val D'orcia
How large will your dinner party be? If it is 4 people or less, I would recommend Il Rosselino in Pienza. It is a small place, with just 4 or 5 tables, run by an older couple. If you tell them you are coming to celebrate a birthday, I am sure they will take extra good care of you.
I would also recommend doing a search of this board (go to upper right-hand corner of this page) for Pienza, Montalcino and Montepulciano. There have been a number of past threads discussing dining in these towns and I am sure you will find many helpful suggestions already here for you to review.
Ristorante Il Rossellino
Piazza di Spagna, 4, Pienza, Italy
If you're going to celebrate with wine and would like to stay in Monticchiello, the Taverna di Moranda is excellent -- we prefer it to the better-known La Porta. If you don't mind driving, da Fiorella in Pienza would be my choice. It's also small, probably 9 or 10 tables, and as at Il Rossellino, I think they would make a birthday ("compleanno") a special occasion. Whatever you decide, reservations are a must.