Altebrilas:Is it possible, to the interrogative form?

Baker: You will eat the cake I make when... [inaudible, e.g. ...you can afford it]
Boy: I will eat the cake you make when?

It would be written as "I will eat the cake you make when...?" because you are prompting a response. You are essentially repeating what you heard the other person say so that they can repeat the rest of what they said. It's not actually a sentence itself (and also not considered a very polite way to go about asking them to repeat what they said - it's not exactly rude, but it's very casual). If you wanted to ask what the rest of the sentence was, you could say "Sorry, what did you say?" or "When will I eat the cake?" - which are fine in both polite and casual circumstances.