From “Mike E. Clark’s Extra Pop Emporium”, which was one of the 3 bonus albums provided with Mighty Death Pop.

Apparently it was supposed to be a collaboration:

“Birfday Party” originally had plans of including Das Racist, but despite Heems liking the idea of the track, Kool A.D. decided it wouldn’t make for a good idea.

https://en.wikipedia.org/wiki/Mike_E._Clark's_Extra_Pop_Emporium

So ICP just put it on the bonus album as an “outtake”. The song turned out perfect the way it is, tho.