2 cups (470 milliliters) peeled hazelnuts, toasted, chopped, and cooled
4 cups (950 milliliters) icing sugar
3/4 cup (180 milliliters) unsweetened cocoa powder
3/4 teaspoon (3.75 milliliters) salt
5 large egg whites, at room temperature
1 tablespoon (15 milliliters) vanilla extract
Sea salt flakes, to garnish
Preheat oven to 325 F (160 C).
In a large bowl, combine hazelnuts, icing sugar, cocoa powder, and salt. Add the egg whites and vanilla and mix until combined.
Spoon roughly two and a half tablespoons (37.5 milliliters) of batter onto parchment paper-lined baking sheets, making sure to leave lots of space between cookies as they expand significantly in the oven. Sprinkle the top of each cookie with sea salt flakes.
Place in the oven for 12-14 minutes, until the surface of the cookies are shiny and they have begun to crack. Transfer to a wire rack and let cool.