“What cake is as hard as a rock?”/“Marble cake.”

A marble cake riddle is:
 
Q: What cake is as hard as a rock?
A: Marble cake.
 
The riddle has been cited in print since at least 1976. “What cake would a sculptor use? Marble” was printed in a 1903 newspaper.
 
 
27 May 1903, Wilmington (DE) Journal, “A Cake Contest,” pg. 2, col. 5:
What cake would a sculptor use?
Marble.
 
13 December 1935, Reading (PA) Times, “Cake Guessing Begets Laughs,” pg. 30, col. 1:
What is the variety cake?
Marble.
   
Google Books
Biggest Riddle Book in the World
By Joseph Rosenbloom
Illustrated by Joyce Behr
New York, NY: Sterling Publishing Company, Inc.
1976
Pg. 96:
What cake is as hard as a rock?
Marble cake.
 
24 February 1976, Aiken (SC) Standard, “Blondie” comic strip, pg. 10, col. 5:
Dagwood Bumstead: THIS SPONGE CAKE IS HARD AS A ROCK!
Cook: IF YOU THINK THAT’S HARD YOU SHOULD TRY OUR MARBLE CAKE!
   
17 June 2007, The Pantagraph (Bloomington, IL), “Riddle Me This,” Flying Horse, comics sec., pg. ?, col. 6:
Q: What cake is hard as a rock?
A: Marble cake.
 
Twitter
Susan Endriss
‏@SuzzzyGal
#GOODHUMORMONDAY:  MARK:  What cake is as hard as rock?  RYAN: It’s unknown to me.  MARK:  Marble cake.
6:25 AM - 7 Feb 2011
   
Houston (TX) Chronicle
Jokes for children
December 24, 2014
(...)
What cake is hard as a rock?
Marble cake.