Why use 'au' instead of 'de'
With the phrase: Tu as un sandwich au chocolat..
why is it not: Tu as un sandwich de chocolat..
Surely de means of, I'm not sure why au is used here, is it an exception?
This website has a pretty good explanation: http://french.about.com/library/prepositions/bl_prep_a_vs_de.htm
It pretty much describes "à" as being used when something is a very important ingredient, where as "de" is used when it is an absolutely necessary ingredient.
"Un sandwich au chocolat" without chocolate is still a sandwich, but "jus d'orange" without oranges is nothing.