“What’s the best thing to eat with jacket potatoes?”/“Button mushrooms.”

A riddle about “jacket” potatoes and “button” mushrooms is:
 
Q: What’s the best thing to eat with jacket potatoes?
A: Button mushrooms.
 
The joke has been cited in print since at least 1978.
   
   
Wikipedia: Baked potato
A baked potato, or jacket potato, is a potato that has been baked for eating. When well cooked, a baked potato has a fluffy interior and a crisp skin. It may be served with fillings and condiments such as butter, cheese or ham.
 
Google Books
The Crack-a-Joke Book
Harmondsworth, UK: Penguin Books
1978
Pg. 108:
What vegetable should you pick to go with jacket potatoes?
Button mushrooms.
 
Google Books
Father Christmas “It’s a Bloomin’ Joke” Book
By Raymond Briggs
London, UK: Puffin
1994
Pg. 70:
What vegetable goes best with jacket potatoes?
Button mushrooms.
 
18 December 1996, The Sun (London, UK), pg. 6:
What’s the best thing to eat with jacket potatoes? Button mushrooms!
     
Twitter
Anna‏
@loveholic198
What vegetable goes well with jacket potatoes? Button mushrooms!! *shot again*
2:25 AM - 30 Nov 2010
 
Google Books
The Bumper Book of Very Silly Jokes
By Macmillan Children’s Books
London, UK: Macmillan Publishing Limited
2013
Pg. 80:
Which vegetable goes best with jacket potatoes?
Button mushrooms.
 
Twitter
Active NT‏
@active_NT
Lunchtime Titter: Which vegetable goes best with jacket potatoes? Button mushrooms! #lovethefruitandveg
6:31 AM - 19 Oct 2016