Batman: Madness A Legends of the Dark Knight Halloween Special (DC, 1994 series) and
Batman: Ghosts Legends of the Dark Knight Halloween Special (DC, 1995 series).
They would also do Batman: The Long Halloween (DC, 1996 series).
Notes:
In the costumed party one can see Bill Watterson's Hobbes, Alice in Wonderland, L. Frank Baum's Tin Man, Jeff Smith's Phoney Bone, Dave Sim's Cerebus, Evan Dorkin's Milk & Cheese, and David Boswell's Reid Fleming.