Evidence from a study in 2012 showed that cocoa flavanols may help to reduce the rate of memory loss, a finding which is of particular importance given the rise of cognitive issues like dementia in the world today. Whilst more research needs to be carried out to establish the link, this early test is promising.
Flavanols are not specifically found in cocoa, appearing in fruit like apples and grapes, as well as tea- in fact, it is one of the reasons for green tea's growing popularity as a health drink. Cocoa is particularly rich in flavonoids however. Flavanols have also been studied with regards to their effects on heart disease and strokes.
The study had 90 people with mild thinking problems drink varying amounts of cocoa for eight weeks, with other flavanol rich foods being restricted to ensure a more accurate measurement of the dose they were receiving. Throughout the two months they participated in a number of tests looking at memory function and brain processing speeds. It was found that those who had consumed cocoa achieved better scores on the tests, with those consuming higher amounts performing even better.
Consuming cocoa as a drink is seen as the healthiest way to benefit from it, as whilst you still gain the benefits from, say, a bar of chocolate ( one with a high cocoa content, mind) the extra fats and sugars used to make the bar can be detrimental to your health.