Cornflakes are a type of breakfast cereal.
In 1991, while Vernon Dursley took them on the run from wizards, the Dursley family and Harry Potter ate stale cornflakes and cold tinned tomatoes. for breakfast while staying at the Railview Hotel.
There exists a spell that causes one's skin to look as though it is coated in cornflakes.
- Harry Potter and the Philosopher's Stone (First appearance)