"Which of the two above calipers is a cake"
This was sorted , legally, in a recent High Court decision, I believe.
Leave both exposed to the atmosphere for a number of days. If one becomes hard, that is a cake. If, however, one becomes soft, that is a biscuit. This decision was required to define whether Jaffa Cakes were cakes or biscuits for tax purposes. 
rgds
Bill