"A study of 60 elderly people with no dementia found two cups of cocoa a day improved blood flow to the brain in those who had problems to start with."
There was no randomized control group in this study, nor even a non-randomized control group. It has a pre-post design:
"[R]esearchers asked 60 people with an average age of 73 to drink two cups of cocoa a day - one group given high-flavanol cocoa and another a low-flavanol cocoa - and consume no other chocolate."
Time passed, lives were lived, and people drank cocoa. Then, the researchers attributed changes to the drinking of cocoa:
"Study author Dr. Farzaneh Sorond, a neurologist at Brigham and Women's Hospital and assistant professor of neurology at Harvard Medical School, said chocolate seemed to boost the brain's blood supply, citing an 8.3 percent increase in blood flow after a month's worth of hot cocoa...'In people with impaired blood flow, she added, "cocoa may be beneficial by delivering more fuel.'"
This research design is not up to the inferences and recommendations being made. It is a flimsy foundation for any medical advice. Yet that is how it is being sold: "Chocolate is the New Brain Food, "Cocoa Can Prevent Memory Loss."
Crappy science reporting is often the dangerous offspring of a press office that writes a sexy, misleading press release and lazy reporters who swallow it whole. A researcher can lose control of the message. The quotes above indicate that, in this case, the researcher is also complicit.