Birthday Cake Rec- South End/Back Bay
There are three places in the South End to get baked goods as far as I know: the South End Buttery, Flour and Aroa. Of the three, I don't think that the Buttery does special order cakes but you may want to double-check. While their baked goods are great, the problem with the Buttery is the hit-or-miss nature of the service--sometimes you get great help otherwise you get uninformed and petulent help (me: "What is that pastry?" counter-person: "It's a chocolate-someting, do you want it or not?").
Both Flour and Aroa are fabulous and you can't go wrong with either one.
I know of three bakeries in the Back Bay: the one in the Shaw's supermarket, a cupcake bakery whose name escapes me for the moment but it's on Mass Ave and Finale. Of the three in the Back Bay, Finale is your best bet.