How to Make It
Preheat the oven to 350°. Line 3 large baking sheets with silicone liners or lightly buttered aluminum foil (press out any wrinkles). In a medium bowl, mix all the nuts.
In another medium bowl, whisk the melted butter with the sugar, cocoa powder and salt. Add the egg whites and whisk until smooth, then whisk in the flour and the grated Parmigiano-Reggiano.
Spoon level teaspoons of the batter onto the baking sheets, about 3 inches apart. Using the back of a spoon, flatten the batter into 2 1/2-inch rounds. Sprinkle each round with about 1 teaspoon of the mixed nuts.
Bake one sheet of cookies at a time for about 12 minutes, until they are just firm. Let the cookies cool on the baking sheet for 1 minute, then transfer them to a wire rack to cool. To make curved cookies, carefully drape the hot cookies over a rolling pin and let cool until they are hardened, about 2 minutes. Bake the remaining cookies and let them cool completely before serving.