Release from the institution that did the research: http://newsroom.cumc.columbia.edu/blog/2014/10/26/flavanols-...

TL;DR:

- "37 healthy volunteers"

- "cocoa flavanol-containing test drink prepared specifically for research purposes was produced by the food company Mars"

- "Most methods of processing cocoa remove many of the flavanols found in the raw plant"

- "The high-flavanol group also performed significantly better on the memory test. “If a participant had the memory of a typical 60-year-old at the beginning of the study, after three months that person on average had the memory of a typical 30- or 40-year-old,” "

- "The researchers point out that the product used in the study is not the same as chocolate, and they caution against an increase in chocolate consumption in an attempt to gain this effect."