Answer to "whose cake is this"?

Answer 1

If you see a cake and you want to know whose cake it is, you would say, "Whose cake is this?"

If someone brings a cake as a present for someone else, and you want to know whom it's for, you can casually ask, "Who is it for?"

Or you can ask very formally, "For whom is this cake?"

"Whose cake is this for" doesn't make sense.