Bring an assortment of Hershey's if you want to show how gross American chocolate can be. That stuff tastes like it's full of wax. You'll just confirm every stereotype about American food quality.

The peanut butter suggestion is a decent one as peanut buttery stuff is hard to find most places in Europe.

However, unless you're quite sure your friends will like American chocolate, that is not the item I would bring to them as a gift. Most American chocolate tastes so cheap and awful compared to European stuff at the same price range.