These lively little cookies, named for Vulcan, the Roman god of fire, have a cracked, rocky surface and crackle like a volcano as they cool. They are made in the central region of France, famous for its lava-dotted landscape. The batter is crunchy with hazelnuts and flavored with cocoa, and it must stand for at least three hours before baking. In compensation, the “stones” will keep for weeks in an airtight container. To ensure crispness, the batter uses baker’s ammonia (sold in baking supp