Well, brownies are probably a more
concentrated source, and they are generally also not quite as well-cooked (in terms of both temperature and time)-- an itchy mouth is one of those things that we were told to put up with as we went along.
I have habitually (over the past 18 months) OVER-cooked our batches of brownies-- I cook them about 30 minutes, and standard time is only about 20. Even at 30, sometimes the center feels (to me) uncomfortably 'not-well-enough-cooked' and I wind up only using the OUTER 2/3's of the pan as doses. DH and I just eat the center.
Unless it's more than itchiness, or it goes on for longer than a few minutes (DD initially found that she would have oral itching/tingling for as much as 30-40 minutes each day), I wouldn't worry too much about it. (But that is just me, and DD has always been on a daily dose of antihistamine to begin with, fwiw.)
Does that help?