In a food processor, finely grind hazelnuts. In a medium bowl, whisk the flour with the ground nuts, baking soda and salt. In a standing electric mixer fitted with a paddle, or using a hand mixer in a large bowl, beat the butter with the sugar at medium speed until light and fluffy, about 3 minutes. Beat in the egg yolk and vanilla, scraping the side and bottom of the bowl. Beat in the dry ingredients at medium-low speed. Divide the dough in half. Pat each half into a round, wrap them in plastic and refrigerate for 15 minutes.
Preheat the oven to 350°. Position the racks in the upper and lower thirds of the oven. On a lightly floured surface, roll out each piece of dough 1/8-inch thick. Using a 2-inch biscuit cutter, stamp out rounds as close together as possible. Arrange the rounds on large baking sheets, about 1 inch apart. Gather the scraps and reroll, cutting out more cookies.
Bake the cookies in batches for about 20 minutes, until golden; shift the pans from top to bottom and front to back halfway through baking. Let the cookies cool on the sheets for 5 minutes, then transfer to racks to cool completely.
Sandwich the cooled cookies with Nutella or another chocolate spread.
The cookie dough can be wrapped in plastic and aluminum foil and refrigerated for up to 1 week or frozen for up to 1 month. Thaw in the refrigerator before using. The baked cookies can be stored in an airtight container for up to 1 week or wrapped in plastic and foil and frozen for up to 1 month.