'cake' was one of DS5's (hes autistic too) first signs as well! it took him 18mths to start signing back to me, but now 6mths later, hes got about 80 signs - im sure it wont be long till your DD is churning them out too! well done your DD!!!
He started out just by watching Mr Tumble! I struggle to stay ahead of him now - he picks them up so quickly - I have a couple of makaton books (from BIBIC) & we're organising makaton training for us/tutors/CM/grandma etc through our ABA consultant.
He's using verbs now & has started putting signs together (e.g. monkey drawing paper to describe what happens in his favourite video).