I've had no problem with chocolate candy, cookies, croissants, etc. I check yes on the form and write in what the food is. They might ask if I have any other food, but no big deal.

However, any fruit, vegetables, meat, cheese, etc. can be a very big deal and lead to a big fine if caught. Sometimes these items are inspected and approved, such as tulip bulbs and flowers from Holland, but the risk to agriculture and health could be nontrivial from some food products. Please don't try to do it. (BYW, Australia is much more strict about this than the USA.)