As mentioned on Wikipedia, in some later editions of The Princess Bride, the first chapter of Buttercup's Baby, which Goldman said was a sequel, appears at the end. A sequel was never realized, and one might argue that he was never serious about Buttercup's Baby, though my impression from an interview (see also Wikipedia link) was that he seriously did want to write a sequel.
Is there any evidence that he wrote more of Buttercup's Baby than 1 chapter?
All of Goldman's "real-world" stories about The Princess Bride (e.g., things about Morgenstern) are made up. Is there any evidence that Chapter 1 of Buttercup's Baby was really intended to be the beginning of the sequel he wanted to write?
Note: I read the 25th anniversary edition, but do not have the 30th, which according to Wikipedia has hints about a sequel plot, so I do not know what is in there, or if that was serious or just fantasy.