How to Make It
Preheat the oven to 325° and position a rack in the center. Spread 1/4 cup of the hazelnuts in a pie plate and toast for about 15 minutes, until the skins blister. Transfer the hazelnuts to a kitchen towel and rub vigorously to remove the skins. Finely chop the hazelnuts.
In a food processor, finely grind the remaining 1/2 cup of hazelnuts with 2 tablespoons of the sugar. Add the flour, cinnamon, allspice, cloves, salt and the remaining 1/2 cup plus 2 tablespoons of sugar and pulse to combine. Add 2 sticks of the butter and pulse until a dough forms. Transfer the dough to a work surface and knead 2 or 3 times. Pat the dough into two 8-inch disks, wrap in plastic and chill until firm, at least 30 minutes.
Line 2 large baking sheets with parchment paper. On a lightly floured surface, working with 1 disk at a time, roll out the dough to a 10-inch round, 1/4 inch thick. Using a 1 1/2-inch round cookie cutter, stamp out rounds as closely together as possible. Arrange the cookies on a baking sheet, about 1/2 inch apart. Repeat with the second disk of dough. Gather the scraps from both batches and pat into a disk. Chill for about 15 minutes, then cut out a few more cookies. Don't use the dough scraps again.
Bake the cookies 1 sheet at a time for about 20 minutes, until the bottoms are lightly colored but the tops are still pale. Transfer the baking sheet to a rack to let the cookies cool.
In a medium saucepan, melt the milk and semisweet chocolate with the remaining 6 tablespoons of butter over very low heat, whisking until smooth. Transfer the chocolate sauce to a medium bowl and let it cool for 10 minutes.
Turn half of the cookies bottom side up. Spoon a small dollop of the chocolate in the center of each cookie. Dip the remaining cookies halfway into the chocolate and sandwich over the bottoms, pressing to seal. Sprinkle the chopped toasted hazelnuts on the chocolate and let the sandwich cookies stand for about 30 minutes, until the chocolate is set.