birthday dinner in Val D'orcia
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.
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