A thin layer of marzipan placed between a shortbread base and a jam filling serves two purposes: it acts as a moisture barrier, preventing the jam from softening the biscuit over time, and it adds a subtle almond flavour that rounds out the overall taste without dominating. Prue Leith's praise was precise — the marzipan you get, but you can't taste it. This is the target: rolled to approximately 2mm, the marzipan provides structure and background flavour as a whisper, not a shout. Too thick and it overwhelms; too thin and it provides no barrier. Cut the marzipan with the same cutter as the biscuit base for a clean, even fit. This technique applies to any filled biscuit or tart where a wet filling risks softening a delicate pastry base.