The earliest version of Cookie Monster appeared on a snack commercial in 1966. Jim Henson brought him to Sesame Street just a few years later, where he has remained ever since. He is among the most successful Muppet characters of all time whose appeal has lasted for generations.
This is, I think, because Cookie Monster speaks to the primal nihilistic urge of modern man to consume and be consumed by the universe. The real Cookie Monster is always inside of us, seeking to satisfy a hunger from which we can never escape but nonetheless are destined to seek.
This leads us to the inevitable question: what are those cookies that Cookie Monster devours so ravenously, as though they were the children of Cronus?
Sopan Deb writes in the New York Times (sorry, it's a paywalled article) about his quest for an answer to this question. They are made of pancake mix, puffed rice, Grape-Nuts, instant coffee, and water. The chocolate chips are made of colored glue.
Muppet wrangler Lara MacLean, who has worked for the Jim Henson Company since 1992, bakes them at home. She prepares the cookies so that they crumble in Cookie Monster's mouth at just the right consistency, letting him feel every crumb of defeat fall from his mouth to the amusement of his young audience.
Cookie Monster's muppeteer, David Rudman, comments, "The more crumbs, the funnier it is."