What Bizznessman says is correct. Now, if you cook as a private chef, you have more leeway, other than getting sued, there is no protection against that. If you are cooking food that is provided by the host, in the hosts facility, you are fairly well-protected from most of the health and welfare violations a normal operator would incur. However, if you take the money, you do need to report it as normal income.
All of that being said, you are better off doing it right, and following all of the proper regulations, if this is a one off, you can get event insurance and be protected. Not cheap, but, worthwhile.