From the Justice League Unlimited episode, "This Little Piggy":
The scenario: Circe (special guest star Rachel York) has turned Wonder Woman into a pig. The Batman (Kevin Conroy), in order to restore the Amazing Amazon to normal, does something rather, well...............unusual, for him. The reactions of Circe and Zatanna are priceless.
Oh, one other thing. Circe's cover in this story was that of a cabaret singer, which allowed Rachel York a rare opportunity to sing, as well. I'll see if I can pull that up sometime.
The point of the plot? As I noted in reviewing Justice League Unlimited, the writers tried to create a romance between Batman & Wonder Woman, although most fans would've preferred a Wonder Woman-Superman coupling (contrary to the comics, Supes wasn't married to Lois Lane on this show). Just for good measure, a decade earlier, producer Paul Dini had established that Zatanna was a childhood sweetheart of Bruce Wayne's, at least to an extent.
Anyway, you may never listen to Cole Porter the same way again after hearing this cover of "Am I Blue"......