These fun little cookies – Scories De Vulcans or Lava Stone Cookies, named for Vulcan, the Roman god of fire, have a cracked, rocky surface and crackle like a volcano as they cool.
Heat the oven to 375°F. 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.
Set aside 2 tablespoons of the remaining sugar. Whisk 1 egg white with the remaining sugar until foamy, add it to the nut mixture in the processor, and work for a few seconds until mixed.
Whisk the remaining egg whites in a bowl until soft peaks form.
Gradually add the reserved 2 tablespoons sugar to the whites and continue beating until a very stiff meringue forms, 1 to 2 minutes.
Add the nut mixture to the meringue and fold them together.
Cover and refrigerate for at least 3 hours and up to 1 day.
When ready to bake, heat the oven to 375°F. Line a baking sheet with parchment paper. Using 2 teaspoons, set rough balls the size of a small walnut on the paper. Bake until puffed and lightly browned, 14 to 17 minutes. Let cool slightly on the paper—they will crackle like a volcano!
Transfer to a rack to cool completely. When cool, the cookies will be light and crisp and have a cracked, rocky surface. They may be stored in an airtight container for a month or more.