I may get some flack for mentioning Magnolia around here, but any time I get dragged there by a friend, I always get their Peanut Butter Ice Box Pie, I personally don't think their cupcakes are anything special, but that pie does it for me every time.
Not sure your price range but the peanut cake from Jean Georges is to die for. If not For the record, the cup cakes at Hill Country are 6 bucks a piece. Also the peanut butter cookies at Magnolia are good too. Perhaps not suitable for a birthday but great none the less..