I had a bantam roo who's favorite food in the world was chips ahoy cookies! lol He's perfectly fine. Your roo probably actually wanted the peanut butter! lol My chickens loooooove peanut butter!
I had a dog that ate a dozen whoopie pies last year (darn thing pushed out the chair and climbed on the table) so here's what I know: the chemical that makes them sick is theobromine. The darker the chocolate is, the more theobromine it has in it. Cocoa powder and bakers chocolate have the most. Milk chocolate has the least. a dog would have to eat 1 ounce of milk chocolate for every lb of body weight. If that holds true for a chicken also....your roo is probably about 4 lbs, which mean he would have had to eat 4 ounces of chocolate. Of course, I'm not positive the dog dose translates to chickens. Keep in mind that even 1 bite of arsenic will not kill you....it may make you sick, but it wont kill you.
Silkiechicken is correct. In moderation it should be just fine.