In a food processor, grind ½ cup of the hazelnuts to a paste. Scrape into a small bowl and set aside.
In the food processor, using the on/off button, grind the remaining 1 ¼ cups hazelnuts with ½ cup of the flour to make a coarse nut flour; transfer to a bowl.
Grate the chilled chocolate in the food processor fitted with the grating blade.
Position a rack in the middle of the oven. Preheat the oven to 350 degrees Fahrenheit. Line 2 large cookie sheets with parchment paper.
Into a bowl, sift together the remaining 1 ¾ cups flour and the salt.
In the bowl of a standing electric mixer, cream the butter and sugar on medium speed until light and fluffy. Add the egg yolks, one at a time, beating well after each addition. Beat in the hazelnut paste and blend well. Add the vanilla, nut flour, grated chocolate, and the sifted flour, mixing well after each addition. Gather the dough into a ball with your hands, and shape it into a disk. Wrap in plastic and chill for 1 hour.
On a lightly floured board, roll out the dough ¼ inch thick. With a round cookie cutter, 2 ¼ inches in diameter, cut out cookies and place them 1 ½ inches apart on the lined cookie sheets. Reroll the scraps only once; flour the rolling pin only if the dough starts to stick to it.
Bake the cookies for 15 to 20 minutes, turning the cookie sheets halfway through baking front to back. Remove from the oven when the edges of the cookies are just starting to brown. Cool the cookies on the sheets, set on racks. Store in an airtight container.