"Here. Eat it. It'll help."
-- Professor Lupin to Harry, handing him chocolate after a Dementor made him faint (PA5)
Chocolate is a very popular sweet food which has magical properties. Not only does it make a wonderful treat, but it also serves as a particularly powerful antidote for the chilling effect produced by contact with Dementors and other particularly nasty forms of Dark Magic. Lupin carries chocolate with him on the Hogwarts Express (PA5), which seems to suggest that he expected a problem with the Dementors, or perhaps that any Defense Against the Dark Arts specialist carries chocolate as a matter of course.
Madam Pomfrey, when she heard that Lupin had given Harry chocolate after his encounter with the Dementors, nodded approvingly and stated that “at last we have a Defence Against the Dark Arts professor who knows his remedies.” She herself uses enormous blocks of Honeydukes’ best chocolate as an antidote in the Hospital Wing (PA21).
Hermione uses chocolate cakes filled with Sleeping Draught to knock out Crabbe and Goyle so Harry and Ron could switch places with them via Polyjuice Potion (CS12)
Commentary
Etymology
Spanish, from New World Yucatec Mayan word "chokol" meaning hot, and the Nahuatl "atl" meaning water
Notes
Chocolate is a product of the plant Theobroma cacao, the fruit of which provides cocoa beans. When those are processed we get cocoa butter, cocoa powder, and chocolate liquor. The indigenous people of Mexico and Central America, the Mayas, Olmecs and Aztecs, prepared cocoa as a sacred drink. They believed their feathered snake- god Quetzalcoatl was thrown out by other gods for giving chocolate to humans. The beans were so valuable they were used as currency. Eventually Spanish explorers took cocoa back to Europe, and the rest, as they say, is history. Chocolate is now an almost universal treat, whether your favorite is Godiva, Cadbury, Hershey's or Belgian. source: Wikipedia
And maybe Professor Lupin was onto something - chocolate is now considered a "super-food" full of antioxidents and flavonoids that might help lower blood pressure and lower bad cholesterol, or even fight cancer cells in the body. source: University of Michigan