You have to tread very carefully here. The opportunist (thieving) neighbour needs to know that their behaviour isn’t acceptable but you also have to be wary of defaming them publicly and of creating unnecessary alarm within the building.
On the question of defamation, given there were no other witnesses, this would just be a “he said, she said” argument in the unlikely even the police were called, but it could lead to an expensive civil action if you reported in minutes that Bloggsy in Number 13 was a kleptomaniac.
You could send a note around that there is a thief in the building and for owners to make sure that no goods were left on doorsteps. But would that raise unnecessary alarm?
However, don’t you have a duty of care to other owners to let them know that the block is not as safe as people might hope or assume?
And as for the cake-taker, they need to know that other people in the building are aware of their shenanigans.
I think if it was up to me, I’d send around a note saying something like:
“An item was taken from the doorstep of a resident recently. The culprit was identified and the item returned. However, please be careful about any deliveries that you may arrange to be left outside your door in your absence. And please report any missing goods to a committee member and we will pursue this with the police if need be.”
The ensuing chatter should be enough to make your sweet-toothed sneak think twice before doing it again. But, sadly, that loss of trust is a bell that can’t be unrung.