A Halloween ghost pun on Post Toasties cereal is:
 
Q: What do ghosts eat for breakfast?
A: Ghost Toasties.
 
The “Ghost Toasties” joke has been cited in print since at least 1940, when the cereal was consumed with “evaporated milk” (a second joke). “What do ghosts eat for breakfast?”/“Dreaded wheat” and “What do ghosts eat for breakfast?”/“Scream of Wheat” are a similar breakfast riddles.
 
“What do ghosts eat for lunch?”/“Boo-logna sandwiches,” “What do ghosts eat for lunch?”/“Spookghetti,” “What do ghosts eat for dinner?”/“Ghoulash” and “What do ghosts eat for dessert?”/“I scream” are related Halloween riddles.
       
 
Wikipedia: Post Toasties
Post Toasties is an early American breakfast cereal that is made by Post Foods. It was named for its originator, C. W. Post. It was the Post version of corn flakes, popularized by Kellogg’s.
 
Post Toasties were originally sold as Elijah’s Manna (c. 1904) until criticism from religious groups (and consequent loss of sales) led to a change of name 1908.
(...)
As of August 2016 Post Toasties are listed as discontinued on the PostFoods web site.
         
4 February 1940, Nashville (TN) Tennessean, “Don’t quote me,” pg 2-C, col. 5:
... have you heard about the one about what man who wasn’t there had for breakfast? ... ghost toasties and evaporated milk ...
 
9 February 1940, Ogden (UT) Standard-Examiner, “Sol’s Sunshine and Shadow, pg. 7-A, col. 1:
What does the little man who wasn’t there have for breakfast? Answer: Ghost toasties and evaporated milk!
 
Google Books 
23 September 1940, Life magazine, “Letters to the Editors,” pg. 8, col. 2:
Sirs:
He sleeps in a disappearing bed, his favorite breakfast consists of Ghost Toasties with evaporated milk and when his health is under par his customary remedy is milk of amnesia.
IRENE HEIM
Los Angeles, Calif.
 
30 March 1952, Boston (MA) Sunday Herald, “Letters from Members,” sec. 2, pg. 16, col. 8:
What do ghosts eat for breakfast? Ghost-toasties and evaporated mi;l.
(...)
BARBARA PACKARD (11),
Wakefield.
 
Google Books
July 1955, Boys’ Life, “Think and Grin,” pg. 62, col. 2:
Breakfast menu for ghosts: Ghost toasties and evaporated milk. — Clipper Kniffin, New Canaan, Conn.
 
12 October 1965, San Diego (CA) Union “Fun Time—The Riddle Box,” pg. B8, col. 6:
What do ghosts eat for breakfast?
Ghost toasties and evaporated milk.
 
16 October 1966, Sunday Star (Washington, DC), “Charlie Rice’s Punchbowl,” This Week magazine, pg. 24, col. 4:
Mark Edwards, of Radio Station WLW, in Cincinnati, suggest the following “menu” for Hallowe’en:
 
Ghost-toasties, Ghoul-Aid, Crypt Suzettes
 
OCLC WorldCat record   
Ghost toasties
Author: Michael J Pellowski; Don Robison
Publisher: Worthington, OH : Willowisp Press, ©1986.
Edition/Format:   Print book : Fiction : Juvenile audience : English
 
Google Groups: rec.humor
Casper the friendly ghost
.(JavaScript must be enabled to view this email address)
7/22/93
Do you know what Casper the friendly ghost ate for breakfast:?
Ghost toasties and evaporated milk…
 
Google Groups: alt.tasteless.jokes
Happy Halloween
Robert Nykvist
10/29/94
(...)                       
What’s a ghost’s favorite breakfast?
Ghost toasties with BOOberries.
   
Google Books
Halloween Hoots and Howls
By Joan Horton
Illustrated by JoAnn Adinolfi
New York, NY: Henry Holt
1999
Pg. ?:
What kind of breakfast does Frankenstein eat?
SCRAMBLED LEGS WITH GHOST TOASTIES AND HOT SCREAM OF WHEAT.
 
Google Books
Dr. Speedy’s Jokes and Riddles
By Speedy Gonzalez
San Jose, CA: Writers Club Press
2000
Pg. 86:
Q: What do ghosts eat for breakfast?
A: Ghost toasties with booberries and evaporated milk.
 
Google Groups: alt.humor
What’s a ghost’s favorite breakfast?
Craig Bennett
10/26/00
What’s a ghost’s favorite breakfast?
Ghost toasties with booberries.
 
Google Groups: alt.humor.puns
Halloween Puns of the Weak 10/19/01
Stan Kegel
10/19/01
(...)
What does a ghost eat for breakfast??
Scream of wheat! (Joke of the Day) 
Ghost-Toasties (Gary Hallock)
 
Google Books
Puns Spooken Here:
Word Play for Halloween
By Richard Lederer
Illustrations by Jim McLean
Layton, UT: Wyrick & Company
2006
Pg. 28:
What’s a ghost’s favorite breakfast cereal?
Ghost Toasties covered with evaporated milk (Isn’t that a cereal killer?)
   
Twitter
Albert Cole
‏@AlbertswagCole
Q: What do ghosts eat for breakfast on Halloween?
A: Shrouded Wheat. Ghost Toasties. Scream of Wheat. Terr-fried eggs. Rice Creepies!
1:09 PM - 15 Oct 2012
 
Twitter
Organic Week
‏@organicweek
Happy #Halloween!
What do goblins eat for breakfast? Ghost Toasties, Scream of Wheat, and Brain Muffins.
HAHA! We’re here all day! 😉
7:45 AM - 31 Oct 2015