Ah so that's who he is, thanks.
His Cookie is really good and I think he's getting into it a bit more. The only way I can tell who does Cookie, Frank or David, is buy the performance. Frank does a lot of stuff that seems like ad lib and he also does a lot of overacting which is funny. Frank's Cookie Monster acts a lot like Piggy in GMC during that scene where she was "hamming it up".
David seems to have started doing that lately too, which is great. Soon the two Cookies will be hard to tell apart.
With David as Cookie, and Eric as Bert, and Frank as Grover there could be a whole scene between 3 of Frank's best SS characters.