I believe using the phrase *because* I'm Batman
is original to HISHE. There are plenty of instances where Batman has said I'm Batman
both in comics and movies. And there was Frank Miller's I'm the goddamned Batman.
Tv Tropes (Danger Will Robinson!) has lots of instances of him saying I'm Batman
across variable medium, but it is only in HISHE that he uses the exact phrasing because.
In the HISHE Superman episode, Batman says "cause I'm Batman." This is the introduction of the Super Cafe with Superman and Batman.
Superman: I know. You say that a lot.
Batman: I carry things on my belt. Cause I’m Batman.
Superman: Dude, it's cool.
The first time in HISHE that Batman says "Because I'm Batman" is in his next appearance, in the Spider-Man 3 episode.
Batman: ... Because I'm Batman.
Superman: Dude, it's cool.