A girl inside a cake was a popular surprise at 1950s’ bachelor parties. A joke variation is:

Q: What happens at a bulimic’s bachelor party>
A: The cake jumps out of the girl.

Google Books
Painfully Gross Jokes (Volume 8)
By Julius Alvin; Miller
New York, NY: Kensington Pub. Co.
1989
Pg. 159:
What’s different about a bulimic’s birthday party?
The cake jumps out of the girl.

1 October 1989, New York (NY) Times, “Accountants Play for Laughs: Did you hear the one about the auditor?” by Alison Leigh, pg. F8, col. 3:
“You know the highlight of a bulimic’s birthday party?” he asked, changing the subject only slightly. “The cake jumps out of the girl.”

11 June 1998, Rolling Stone, “Beastie Boys Get Nasty” by Joe Levy, pg. 25:
And then there were the jokes: “How do you know when you’re at a bulimic’s bachelor party? The cake jumps out of the girl.”

Google Books
A Prairie Home Companion
Pretty Good Joke Book
By Brian Becker
Minneapolis, MN: HighBridge
2003
Pg. 152:
What’s different about a bulimic’s birthday party?
The cake jumps out of the girl.

Google Books
The Playboy Guide to Bachelor Parties:
Everything You Need to Know About Planning the Groom’s Rite of Passage
By James Oliver Cury
New York, NY: Fireside
2003
Pg. 178:
QUESTION: Did you hear about the bulimic bachelor party?
ANSWER: The cake jumped out of the girl!

19 March 2004, Herald Sun (Melbourne, Australia), “Eating a sick joke” by Fay Burstin, pg. 32:
Did you hear about the bulimics’ convention? That’s where the cake jumped out of the girl.

5 June 2004, Herald (Newcastle, New South Wales), “A weight off my mind; that’s life ...” with Mark Rothfield, pg. 30:
Bulimia doesn’t seem very palatable, either we’ve all heard about the birthday party where the cake jumped out of the girl.

Twitter
Austin Alvarado
‏@skunkhair
“Bulimic bachelor party: cake jumps out of the girl.” -Jackie Martling
5:57 PM - 22 Jan 2009

Twitter
Paul Rodger
‏@player112345
this is m 200th tweet and it’s for everybody...how do you know when you’re at an anorexics party ?... the cake jumps out of the girl
8:12 AM - 24 Apr 2009

Twitter
jann arden
‏@jannarden
Jann’s Twitter Joke: What happens at a Bulimic’s birthday?..... The cake jumps out of the girl…
4:37 PM - 27 Apr 2009

Google Books
Aussie Gags
By John Blackman
Momentum
2012
Pg. ?:
Then there was the bulimic’s surprise party – the cake jumped out of the girl.