Dark chocolate helps boost your mood and sustain clear thinking among adults who are engaged in intense mental efforts — like students cramming, or journalists on deadline.

http://www.npr.org/blogs/thesalt/2014/07/14/329529110/food-mood-connection-how-you-eat-can-amp-up-or-tamp-down-stress

The following is from the above web link.

DREW RAMSEY, M.D., a psychiatrist at Columbia University and author of The Happiness Diet, says a nutrient-rich diet is best for beating stress.

For dessert, go for dark chocolate, which can have "an acute affect on mood," 

Ramsey says. He points to a study that found cocoa flavanols can help boost mood and sustain clear thinking among adults who are engaged in intense mental efforts — like students cramming, or journalists on deadline.