This poster has me a bit puzzled:
I don’t know where it came from. There was a bit of fan fiction written by someone called Goldy about just this topic and published in 2001, but note the date referred to on the poster. Summer 1998. In the Angel episode I WILL REMEMBER YOU (23 November 1999) Cordelia has a line referring to the film Titanic, but that is the only reference I can find in either Buffy or Angel. So was this poster for a video game or something else?
In 2008 there was some mention during the Virtual World Expo of “Buffy the Vampire Slayer” and “Titanic” being the inspiration for separate virtual worlds, but nothing of a joint nature was mentioned.