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 supply stores and on the Internet), the same curious ingredient used in Croquets de Sancerre. Like ammonia, it stings your nose and must be used in a tiny quantity. For Coffee Stones, substitute 1½ tablespoons instant espresso powder for the cocoa.
Heat the oven to 375°F/190°C. Spread the hazelnuts on a baking sheet and toast them until golden, 8 to 12 minutes. If the hazelnuts were not peeled, rub them while still hot with a rough cloth to remove the skins. Let them cool completely, then grind them very coarsely in a food processor with half of the sugar—crunchy texture is important. Pulse to work in the cocoa powder, flour, and baker’s ammonia.
When ready to bake, heat the oven to 375°F/190°C. Line a baking sheet with parchment paper. Using
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