How did "a piece of cake" come to mean a very easy task?

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: : : The "Random House Dictionary of Popular Proverbs and Sayings" by Gregory Y. Titelman states that: "It's a piece of cake - It's very easy to do. First used in the mid-twentieth century. During World War II, British soldiers used the expression to describe a mission that was extremely easy to accomplish."

: : : However, the "Morris Dictionary of Word and Phrase Origins" by William and Mary Morris (HarperCollins Publishers, New York, 1977) give America the credit for originating the phrase. Mr. and Mrs. Morris also give the origin of "easy as pie."

: : : "piece of cake/easy as pie - The two expressions are remarkably alike in meaning. 'As easy as pie' is an American expression. Back in the 1890s 'pie' was a common slang expression meaning anything easy, a cinch; the expression easy as pie stemmed quite readily from that. A 'piece of cake' has a somewhat more devious history. According to the Oxford Dictionary, it first appeared in print in a work by Ogden Nash, who wrote in 1936: 'Her picture's in the papers now, and life's a piece of cake.' But, if it first turned up in America, it was swiftly adopted by British airmen in World War II. In 1943 the author of 'Spitfires over Malta' wrote: 'The mass raids promised to be a 'piece of cake' and we expected to take a heavy toll.' Certainly 'piece of cake' was more originally more popular in Britain than in the United States."

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http://www.answerbag.com/q_view/38514

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It’s meaning came from the early Egyptians where cake was a synonym for something good or easy. Mummies were often interred with a doggie bag of cakes and ale, and "cakes and ale" is still common shorthand for "the good life" in Britain.

Attributed to the ancient Greeks a "cake" in those times was made with honey. Aristotle is quoted in the "The Knights": "if you surpass him in impudence, then we take the cake".

Writer Ogden Nash, born in Rye, New York wrote the line (piece of cake) in 'Primrose Path' (1936). Nash could have borrowed it from the French - c'est du gateau. Many phrase origins arrived with immigrants.
"Cake-walk" and "Easy as pie" soon followed.

This phrase originates from the tradition of giving cakes as prizes in rural competitions in USA slavery states when slaves or free descendents would walk in a procession in pairs around a cake at a social gathering or party, the most graceful pair being awarded the cake as a prize. This gave us the expressions 'cake walk' and 'a piece of cake’ both meaning a job or contest that's very easy to achieve or win.

Reference Link:

http://www.businessballs.com/clichesorigins.htm