“The hardness of the butter is proportional to the softness of the bread/roll” is a jocular saying that appears on many collections of one-liners. The Washingtonian of 1982 (the Google Books date may be inaccurate) printed the aphorism:
 
“Harriet’s Dining Observation: ln every restaurant, the hardness of the butter pats increases in direct proportion to the softness of the bread. —Harriet Markman”
 
Paul Dickson’s The New Official Rules: Maxims for Muddling Through to the Twenty-First Century (1989) contained Harriet’s Dining Observation, crediting “Harriet Markman; from Steve Markman, Pasadena, Calif.” Arthur Bloch’s The Complete Murphy’s Law: A Definitive Collection (1990) called this “Thiessen’s Law of Gastronomy.”
 
     
Google Books
The Washingtonian
Volume 18
1982
Pg. 106:
Harriet’s Dining Observation: ln every restaurant, the hardness of the butter pats increases in direct proportion to the softness of the bread. —Harriet Markman
 
Google Books
Another Treasury of Clean Jokes
By Tal D. Bonham
Nashville, TN: Broadman Press
1983
Pg. 150:
The hardness of the butter is inversely proportional to the softness of the bread.
 
Google News Archive
26 April 1984, Latah Observer (Troy, ID), pg. 17, col. 5:
The hardness of the butter is in direct proportion to the softness of the roll.
 
6 January 1985, The Register (Santa Ana, CA), “Hardly a trickle of support for Fountain Valley’s fountain” by Jerry Kobrin, pg. H2, col. 4:
“Do not be dismayed when the hardness of the butter is in proportion to the softness of the roll.” 
(Submitted by Viron B. Casey of Anaheim—ed.)
   
Google Books
The New Official Rules:
Maxims for muddling through to the twenty-first century
By Paul Dickson
Reading, MA: Addison-Wesley Pub. Co.
1989
Pg. 89:
Harriet’s Dining Observation. In every restaurant, the hardness of the butter pats increase in direct proportion to the softness of the bread being served. — Harriet Markman; from Steve Markman, Pasadena, Calif.
 
Google Books
The Complete Murphy’s Law:
A Definitive Collection
By Arthur Bloch
Los Angeles, CA: Price Stern Sloan
1990
Pg. 219:
THIESSEN’S LAW OF GASTRONOMY: The hardness of the butter is in direct proportion to the softness of the roll.
   
Google Books
Wise Quotes of Wisdom:
A Lifetime Collection of Quotes, Sayings, Philosophies, Viewpoints and Thoughts
By R.A. Wise
Bloomington, IN: AuthorHouse
2011
Pg. 176L
RULES—Harriet’s Dining Doctrine: In every restaurant, the hardness of the butter pats increases in direct proportion to the softness of the bread being served. –- Harriet Markman