I don't know whether it is the cause of the craving, but chocolate contains dopamine and related chemicals, such as serotonin, but not levodopa ("Simultaneous Determination of Biogenic Amines ... ", Baranowska, 2015). Banana, especially the skin, is also a source of dopamine.
Dopamine is usually considered not to cross the BBB, but eating chocolate seems to make a minor improvement to my PD symptoms. It also has for me a good laxative effect.
John