the magic cookie
I am not sure if this is what you are talking about or not, but on the album Cookie Monster and Grover: True Blue, there is a track called The Magic Cookie, read by Herbert Birdsfoot, in which Cookie Monster goes searching for a fairy to give him magical joy and happiness. He goes into a few dangerous places, like a cave that a witch lives in and something that a dragon lives in. He asks the kids at home what he should do. Eventually, he finds the fairy, who gives him a cookie that will give him joy and happiness, but only if he doesn't eat the cookie. However, he asks the kids what he should do, and he eats the cookie. I am not sure if this is what you are talkign about or if this was done on the show.
Interestingly, there was an old man who appeared at the beginning who told cookie monster about getting joy and happiness who sounded like he was performed by Caroll Spinney (his voice sounded like a slightly older Big Bird), and there was some growling by a dragon who was voiced by Jerry Nelson, and I can't tell who performed the fairy, but it sounds like she had a natual womans voice (as opposed to a man pretending to be a woman).
Kimp The Shrimp mentioned something where cookie monster had to learn to share from a cookie tree. That was a book called Cookie Monster And The Cookie Tree, where a witch who owns a talking cookie tree panics over seeing cookie monster enter the park and puts a spell on the tree so that he only gives cookies to those who share, since nobody thinks that Cookie Monster would share cookies, but eventually the cookie tree refuses to give the witch cookies because she won't share either. You can see scans of every page of this at Tough Pigs in the Tough Pigs Book Club section.