Certainly would :D It can be confusing and you might be asking, "Why doesn't eisiau use yn?" It's because technically the "verb" eisiau is used in the construction "Mae eisiau .... arna i" but nowadays it much more common to see Dw i eisiau.....
If something is "wanting" on you because you need it/lack it, then it is ar someone :)
The conjugation of ar is taught in the course :)
What would "do you want chocolate be?" Apparently this has to be "do you want a chocolate".
'Do you want chocolate?' and 'Do you want some chocolate?' are also accepted as answers.
If either of those has been rejected, check that you are using the latest version of the Duo app. If they are still rejected, raise a 'Duolingo bug report'.
The correct answer shown on this discussion page is "Do you want a chocolate?" Can that be corrected (remove "a") or do you have no control over that?
'...a chocolate' and '...any chocolate' are also correct:
- Dych chi eisiau siocled? - Do you want [a/some/any/] chocolate?