"Das Mädchen isst ein Hähnchen."
It's not a misspelling, both words are correct. The one comes from the male chicken: "der Hahn - das Hähnchen" and the other from the female chicken: "das Huhn - das Hühnchen".
I think we use "Hühnchen" as a general word for chicken, for example when something contains chicken or comes with chicken etc. ... but we say Hähnchenschenkel (chicken drumsticks) or Grillhähnchen (grilled chicken). Though... for chicken salad I'd rather say "Hühnersalat". Hm.
To make the confusion perfect, I think I have to add that the female chicken is also called "die Henne", just like the "hen" in English. Sorry if it seems complicated.. you can stick with "Hühnchen" in case you're confused, people will understand. =)