Well, apparently ...
It's a cake.
Why?
No VAT is paid on cakes or biscuits.
However, VAT is paid on chocolate covered biscuits.
McVitie's challenged the Inland Revenue in Court over the classification of the Jaffa Cake.
The Inland Revenue claimed it was a biscuit (subject to VAT)
McVitie's claimed it was a cake (no VAT)
McVitie's won the caes.
So no VAT is paid on a Jaffa Cake.
Why?
Because it's not a biscuit covered with chocolate ... it's a cake !!
Official !!
Well ... insofar as naythin on "Wiki" is "official".