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.
 
The joke has been cited in print since at least 1989.
     
 
Wikipedia: Bulimia nervosa
Bulimia nervosa, also known as simply bulimia, is an eating disorder characterized by binge eating followed by purging or fasting, and excessive concern with body shape and weight. This activity aims to expel the body of calories eaten from the binging phase of the process. Binge eating refers to eating a large amount of food in a short amount of time. Purging refers to the attempts to get rid of the food consumed. This may be done by vomiting or taking laxatives.
 
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
9:12 AM · Apr 24, 2009
   
Twitter
Jann “stop live horse export” Arden
@jannarden
Jann’s Twitter Joke: What happens at a Bulimic’s birthday?..... The cake jumps out of the girl…
5:37 PM · Apr 27, 2009
   
Twitter
Duke and the Doctor
@dukeandthedoc
Duke attended a bulimic bachelor party recently……. The cake jumped out of the girl!!!!! http://bit.ly/b9JFT0
10:32 AM · Feb 5, 2010
   
Twitter
Bag Raiders
@bag_raiders
did u hear about the bulimic B’day party?  The cake jumped out of the girl..  http://bit.ly/bgPdhl
11:54 PM · Feb 9, 2010
   
Twitter
Frank Dymond 
@FrankDymond
That bulimia party was fun until the cake jumped out of the girl
12:37 AM · Feb 27, 2010
   
Twitter
Colin Wood
@Woodo
My mate hired a bulimic stripper for his stag night. The cake jumped out of the girl…
12:21 PM · Jun 1, 2010
   
Twitter
Marc C.
@BHLC
I went to a Bulimic birthday party tonight. It was awsome. We all ate, then afterwards a cake jumped out of the girl.
5:49 PM · Sep 20, 2010
   
Twitter
Henco Harmse
@_Henco_
Joke in bad tase of the day: I went to a bulimic’s birthday party. The cake jumped out of the girl.
3:03 AM · Feb 9, 2011
 
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.
     
Twitter
Kevin Barbare
@KevinBarbare
Went to a bullemic bachelor party…the cake jumped out of the girl.
10:07 AM · Jan 29, 2013
   
Twitter
Austin
@mraustinbond
Did you hear about the bulimic birthday party where the cake jumped out of the girl?
4:17 AM · Feb 1, 2013
     
Reddit—Jokes
Posted by u/capomatt November 11, 2013
Ever been to a bulimic’s birthday party? (fixed)
The cake jumps out of the girl.
 
Twitter
Montaukoutloud
@Montaukoutloud
I just went to a bulimic bachelorette party. The cake jumped out of the girl.
11:55 AM · May 14, 2015
   
Twitter
Flying Purple People Eater
@fppe
I went to a bulimic surprise party… the cake jumped out of the girl.
1:12 AM · Jan 23, 2017
   
Twitter
Flying Purple People Eater
@fppe
I went to a bulimic surprise party… the cake jumped out of the girl.
10:19 PM · Jan 22, 2019
   
Reddit—Oneliners
Posted by u/BenjaminDefer October 3, 2022
I was at this bachelor party where the stripper was bulimic wich was funny because the cake jumped out of the girl. “Dan Mintz”
 
Twitter
Bob Kostic
@causticbob
Went a bulimic birthday party at the weekend. The cake jumped out of the girl
7:03 PM · May 22, 2023