“Strength is the capacity to break a chocolate bar into four pieces with your bare hands—and then eat just one of the pieces” was written by American author Judith Viorst in Redbook magazine (1975-1777; the date is not certain). A KitKat bar comes in four pieces; a Hershey’s chocolate bar can also easily be broken into four pieces.
 
Similar “you can’t eat just one” sayings include “Betcha can’t eat just one” (potato chips) and “No man in the world has more courage than the man who can stop after eating one peanut” (peanuts).
 
   
Wikipedia: Judith Viorst
Judith Viorst (born February 3, 1931) is an American author, newspaper journalist, and psychoanalysis researcher. She is perhaps best known for her children’s literature, such as The Tenth Good Thing About Barney (about the death of a pet) and the Alexander series of short picture books.
 
1 June 1977, Journal-News (Hamilton, OH), “Got a Minute?” by Bill McDulin, pg. 7, col. 5:
Strength is the capacity to break a chocolate bar into four pieces with your bare hands and then eat just one of the pieces.
   
Google Books
Bulletin
American Dahlia Society
Volume 66, Issue 3 - Volume 67, Issue 3
1979
Pg. 25:
Strength is the capacity to break a chocolate bar into four pieces with your bare hands — and then eat just one of the pieces. — Judith Viorst in Redbook
 
Google Books
A Woman’s Journal
By Running Press
Philadelphia, PA: Running Press Book Publishers
1985
Pg. ?:
Strength is the capacity to break a chocolate bar into four pieces with your bare hands — and then eat just one of the pieces.
-JUDITH VIORST, b. 1931
American poet
 
19 December 1994, Los Angeles (CA) Times, “Advice From the Home(spun) Front‎” by Jack Smith, Life & Style, pg. 1:
A bit of useful philosophy is offered by Judith Viorst: “Strength is the capacity to break a chocolate bar into four pieces in your bare hands and then just eat one of the pieces.”
 
20 May 2000, Grand Forks (ND) Herald, “Keep the liberty and the death, just give me chocolate.” pg. 1:
Judith Viorst, an avowed chocolate lover, said “Strength is the capacity to break a chocolate bar into four pieces with your bare hands—and then eat just one of the pieces.”
 
Google Books
Food and Drink:
A Book of Quotations
Edited by Susan L. Rattiner
Mineola, NY: Dover Publications, Inc.
2002
Pg. 35:
Strength is the capacity to break a chocolate bar into four pieces with your bare hands — and then eat just one of the pieces.
JUDITH VIORST
 
Google Books
Heavenly Humor for the Chocolate Lover’s Soul:
75 Chocolate-Covered Inspirational Readings
By Barbour Publishing, Inc.
Uhrichsville, OH: Barbour Publishing, Inc.
2011
Pg. ?:
Strength is the capacity to break a chocolate bar into four pieces with your bare hands—and then eat just one of the pieces.
JUDITH VIORST