"What are these?" Harry asked Ron, holding up a pack of Chocolate Frogs. "They're not really frogs, are they?" (PS6)
Also, a chocolate frog was useful in fighting off his dementor-boggart when Lupin taught Harry the patronus charm (pa12)
Chocolate frogs are J.K. Rowling's favorite candy as well as Harry's, but only because she would like to collect the cards (BN, Sch2)
The idea for Chocolate Frog cards is supposedly a tribute on JKR's part to Monty Python.
"British comedy is an obsession of mine. I love Monty Python."
-- J. K. Rowling (Sch2)
The effects of chocolate on the brain are documented: http://science.howstuffworks.com/life/inside-the-mind/emotions/chocolate-high2.htm