I don't know if there's an equivalent in French, but perhaps it would be better to keep the English term with an explanation in brackets, as it is already explained in the text.

This sounds like a particularly British term, referring to the layer of marzipan that is put on a Christmas cake before it is covered with white icing.

Some cakes have a layer of marzipan — a paste made from almonds, sugar, and egg whites. This is usually found just below the icing, hence it serves as the underlying (so to speak) metaphor for the head-scratchingly common business phrase, marzipan layer.

http://www.wordspy.com/words/marzipanlayer.asp