I am assured by half the Muppet Experts in the world that Cookie Monster, as seen on The Colbert Report, was performed by a gent named David Rudman. Frank Oz began cutting back on his Muppeteering in the nineties due to age and other career opportunities, and others began imitating him. Around 2001, Mr. Rudman emerged as the primary Cookie Monster manipulator and it's easy to see why. He's darn good at it.
A month or two ago, I spent a few hours on the set of a Muppet shoot with Kermit, Gonzo, Fozzie, Piggy, The Swedish Chef, Pepe the King Prawn and a few others. Four Muppeteers were handling the cast…with a few more providing hands and such. The four were Dave Goelz, Steve Whitmire, Eric Jacobson and Bill Barretta. Dave originated and still plays Gonzo and Bunsen Honeydew, and Bill started Pepe and a few other recent characters.
But for most of the stars, we're on to second (and third) generation Muppet Wranglers. When Jim Henson died in 1990, he willed the role of Kermit to Steve. Eric is the specialist in the roles Frank Oz no longer does — Bert, Grover, Fozzie, etc. For a while, he did Cookie Monster on Sesame Street but Rudman took it over, in part because it was easier to have Bert and C.M. interact if they weren't done by the same performer. I don't watch Sesame Street often and haven't since it essentially turned into The Elmo Hour. But when I've caught new Muppet segments, I am impressed with how faithful they are…and how well they integrate with scenes rerun from the era of Henson, Oz, Hunt, Nelson, et al.
Having been deeply critical of some cast replacements on animated shows, I'm impressed that someone knows the difference between a good Muppet imitation and a bad one…and hires accordingly. The four men I saw on that set a few months ago were as good as you could reasonably expect; maybe a little better than one would dare hope for. I wish more people cared that much about keeping classic characters sounding and behaving like themselves.