The porcini (Boletus edulis), known in English as the king bolete, cep or penny bun, is among the most prized of all wild edible mushrooms — a stout, heavy fungus with a fat, bulbous off-white stalk and a glossy, domed cap the colour of a baked bun, from pale chestnut to deep red-brown. What sets it apart from a field mushroom is the underside of its cap: instead of gills it carries a thick, spongy layer of fine vertical tubes that open as a mass of tiny pores, white when young and turning yellow-green with age. Firm, meaty and intensely savoury, porcini is one of the great umami ingredients of European cooking, and it grows still more concentrated when dried, gaining a heady, almost meaty perfume. Because it lives in partnership with the roots of trees it cannot be farmed and must be gathered from the wild, which is why it remains a luxury — a pillar of Italian and French autumn cooking in risotto, pasta, sauces and stews.