A review of randomized controlled trials found that people generally had enhancements in working memory performance and improved visual information processing after eating cocoa flavanols.Eating cocoa after a night of total sleep deprivation actually counteracted the cognitive impairment that lack of sleep can cause.
One should be cautious about buying any dark chocolate brand because a recent study found many brands of dark chocolate have lead and cadmium levels that exceed safety standards.
Here is a list of the brands that had high or normal levels of lead and cadmium in their chocolate (scroll to the bottom of page). http://www.asyousow.org/%E2%80%A6/toxic-e%E2%80%A6/lead-and-cadmium-in-food/
I find that a cup of strong coffee and a bar of dark chocolate works wonders when driving or playing video games. Not so sure about the cocoa butter…not to mention the cadmium/lead.