Gâteau Périgourdin

Walnut trees do well in the thin, stony soil of Périgord, leading to walnut pastries and cakes, including this moist, rich gâteau with a topping of crisp caramel. Like most nut cakes, this one improves when kept a day or two in an airtight container. But the topping should be added only a short time before serving, as caramel softens after a few hours in the open air.