In the Three Kingdoms Period (sān guó 三国时代,220-280AD), the king of the state
of Wei (wèi guó 魏国), Cao Rui (cáo ruì 曹睿), wanted to select a very capable man
to work for him. He said to his ministers: "When choosing a talented person,
always beware of one with a false reputation. A false reputation is just like a
picture of a cake; it can' t relieve hunger."

Later, this idiom came to be used to mean comforting oneself with unrealistic
thoughts, without solving practical problems.

三国时代魏国的皇帝曹睿，准备选拔一个有才能的人到朝廷来做官。曹睿对他的大臣说：“选择人才，不能光找有虚名的人。虚名好像是在地上画的一块饼，只能看，不能解决肚子饥饿的问题啊!”
后来人们就用“画饼充饥”这个成语比喻用空想安慰自己，不能解决实际问题。

画饼充饥【翻译】Allaying Hunger with Pictures of Cakes.