I think it was number 50 of the Legends of the Dark Knight, called "Images", by Dennis O'Neil and Bret Blevins.
In this story the Joker has a cousin called Melvin Reipan, an idiot savant with a genius for chemistry and the mind of a child. The Joker tricks him into believing that he is hideous and that they'll find a "cure" for him.
The Joker tests the "joker venom", developed by Melvin, on Melvin's cat. He also uses Melvin's body to feign his own death.
This story presents a different version of the first encounter between Batman and the Joker. Batman is able to recall that he heard the Joker's voice before as the Red Hood.