1/4 cup granulated sugar, plus more for sprinkling
2 sticks ( 1/2 pound) unsalted butter, softened
1 1/2 cups confectioners' sugar, sifted
2 large eggs, separated
1 teaspoon pure vanilla extract
2 1/2 cups all-purpose flour
1 teaspoon cinnamon
1/2 teaspoon salt
Preheat the oven to 400°. Spread the hazelnuts on a rimmed baking sheet and toast in the oven for about 10 minutes, or until deeply browned. Let the hazelnuts cool slightly, then transfer them to a kitchen towel and rub vigorously to remove the skins. Let the nuts cool completely, then transfer them to a food processor with the 1/4 cup of granulated sugar. Process just until the hazelnuts are finely ground.
In a large bowl, cream the butter with confectioners' sugar. Beat in the egg yolks and vanilla. Stir in the ground nuts, flour, cinnamon and salt to make a smooth dough. Pat the dough into a disk, wrap in plastic and refrigerate until firm, about 2 hours.
Divide the dough in half and refrigerate 1 half. On a lightly floured surface, roll out the other half to a 12-inch square. Cut the dough into sixteen 3-inch squares. Cut the squares on the diagonal to form 2 triangles each. Arrange the triangles about 1 inch apart on baking sheets and refrigerate until well chilled, about 15 minutes.
Preheat the oven to 350°. In a small bowl, lightly beat the egg whites. Brush the cookies with the whites and sprinkle with granulated sugar. Bake in the center of the oven for about 15 minutes, or until golden. Let stand on the sheets until firm, then transfer to a rack to cool. Repeat with the remaining dough.
The cookie dough can be refrigerated for up to 2 days or frozen for up to 1 month. The baked cookies can be kept in an airtight container for up to 2 days.