I read on BYC once about a bird (not a chicken) choking on PB but from my experience it is OK if the PB is spread thinly on bread. They simply love it.
Gave my hens a peanut butter and jelly sandwich and it went over well! Big globs of peanut butter might not be a good idea, but otherwise I wouldn't worry about it. Just be sure to offer plenty of water to help wash it down.