How would you say this using moyn please? Would it be :
Wyt ti'n moyn siocled?
Yes it would be. The only difference in the sentence structures would be that "Moyn" requires "yn" whereas "Eisiau/Isio" does not.
When to use dych chi'n or dych chi, dw i or dw i'n
As explained in the course notes:
For the uses of 'n/yn, see the course notes for various sections, and look at this discussion - https://www.duolingo.com/comment/13844144