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.
Heat the oven to 325°F/160°C. Toast the bread in the oven until very dry, 6 to 8 minutes. Let it cool, leaving the oven on. Break the cooled bread into pieces and grind it to crumbs in a food processor. Add the walnuts and salt and grind to a coarse powder (the dry bread helps keep the walnuts light). Butter the cake pan, line the bottom with parchment paper, and butter the paper.
Cream the butter in a stand mixer fitted with the whisk attachment. Add half of the sugar and continue beating until the mixture is soft and light, 3 to 5 minutes. Beat in the egg yolks one by one at high speed, beating well after each addition and scraping the sides of the bowl to be sure all the ingredients are mixed. Beat in the lemon zest. Using a spoon, stir in the ground walnut mixture.
Using the mixer with another bowl, whisk the egg whites until stiff. With the whisk turning, gradually beat in the remaining sugar and continue beating until a stiff, glossy meringue is formed, 30 seconds to 1 minute. Fold about one-fourth of the meringue into the walnut mixture to lighten it, then add the walnut mixture to the remaining meringue and fold the mixtures together as lightly as possible. Spoon the batter into the prepared pan.
Bake the cake until it pulls away from the sides of the pan and a skewer inserted in the center comes out clean, 40 to 50 minutes. If the cake browns too quickly, cover it loosely with aluminum foil. Let the cake cool for 5 minutes, then turn it out onto a rack covered with a sheet of parchment paper. Strip the lining paper from the cake and leave it upside down (so it has a flat top) to cool completely, at least 1 hour.
For the topping, put the sugar and water in a small saucepan and heat gently without stirring until the sugar dissolves. Raise the heat and boil until the sugar cooks to a golden brown caramel, 3 to 5 minutes. Turn the cake top upward and set it back on the rack; set the rack on a baking sheet to catch drips. Take the caramel from the heat, let the bubbles subside, and at once pour it over the cake, spreading it with a palette knife to make a very thin layer and letting it drip down the sides. Take care, as caramel can cause serious burns. Decorate the cake at once with the walnut halves so they stick to the caramel. The caramel will become crisp as it cools. When it starts to set, mark portions in the caramel with a knife so the cake is easy to cut into wedges.
© 2007 All rights reserved. Published by Chronicle Books.