A psychologist would ask the jaffa cake whether it feels more comfortable as a cake, or whether it is really a repressed biscuit, and whether it's confusion is due to a bad childhood.
On a serious note, I think the clue is in the name
I believe the definition of a biscuit is that it goes soft when stale, and that a cake will go hard or crisp when stale. So yes a jaffa cake is a cake.