Cocoa beans and processed whole chocolate
In a small study in the journal Nature Neuroscience, healthy people, ages 50 to 69, who drank a mixture high in antioxidants called cocoa flavanols for three months performed better on a memory test than people who drank a low-flavanol mixture.
The findings support recent research linking flavanols to improved blood circulation, heart health and memory and suggests flavanol consumption would enhance memory that would be lost from the normal process of aging.
However the consumption of cocoa flavanols was not associated with increased activity in another area of the brain which is impaired early in Alzheimer’s disease, so consumption of flavanols would not seem to help people suffering from alzheimer.
Most chocolate bars are made by a process called dutching and alkalization that destroys flavanol so chocolate candy bars don’t have a lot of flavanol in them.
To get the benefits, you would have to consume a large amount of chocolate, along with its fat and calories, such as whole chocolate or "chocolate tea" consumed in the Caribbean.
The researchers plan more extensive research. One theory of why flavanols would help memory is that they improve brain blood flow; another theory is that they cause dendrites, the message-receiving branches of neurons, to grow.
Read more here.