Scientists delivered a symposium on the neurobiology of chocolate at this year's meeting of the American Association for the Advancement of Science. There they presented the findings of their research, namely that the flavanols in chocolate help out the brain:
"The beneficial brain effects appear to stem from flavanols' impact on the blood system. In essence, the chemicals stimulate an increase of blood flow to the brain, particularly in areas that light up during tasks that require alertness. Experiments suggest this neurovascular activity is distinct from the well-known stimulant effects of caffeine."The downside is that most chocolates lose their beneficial flavanols during processing. You can read about it all here.