A 2009 study of 30 individuals fed 40g of dark chocolate per day over a 2 week period showed a decrease in urinary output of the stress hormones cortisol and catecholamines.
Another study indicates that the flavanols in dark chocolate help to increase blood flow to the brain and also to lower the stress level in those attempting spacial tasks.
As the researchers put it, “Cocoa flavanol consumption may increase neural [brain] efficiency in spatial working memory function” (Camfield DA et al. 2012)
Remember though, not all chocolate is created equal. Try to go for fair trade and at least 72% cocoa in whatever you purchase.