In a medium bowl, whisk together flour, granulated sugar, baking powder, salt, and cinnamon. In a food processor, pulse hazelnuts with 1/4 cup flour mixture until very finely ground. Add remaining flour mixture and process to combine. Add butter and pulse until mixture resembles coarse meal. Add egg yolks and vanilla and pulse until dough just comes together. Form dough into 2 disks, wrap in plastic, and refrigerate 30 minutes.
Preheat oven to 350 degrees, with racks in upper and lower thirds. Working with 1 disk at a time, roll out dough between floured parchment paper to 1/8-inch thickness. Stack dough in parchment on a baking sheet; refrigerate until firm, 30 minutes. With a lightly floured 2 1/2-inch fluted round cutter, cut out dough. (If dough becomes too soft, refrigerate a few minutes; reroll scraps, if desired.) Place cookies, 1 inch apart, on two parchment-lined baking sheets. With a 1-inch cutter, cut out dough in centers of half the cookies. Bake until cookies just begin to brown at edges, 10 to 12 minutes, rotating sheets halfway through. If desired, bake small cutouts separately, 5 to 7 minutes. Transfer cookies to wire racks and let cool completely.
Lightly sift confectioners' sugar over cutout tops. Spread about 2 teaspoons filling on bottoms of remaining cookies and sandwich with sugar-dusted tops.
Filling Fun: We used melted semisweet chocolate and raspberry and apricot jams. Experiment to find a combo you like. You can sandwich the small cutouts with filling, too.
Disks of dough can be refrigerated overnight or frozen, up to 1 month, and thawed in the refrigerator. Let sit at room temperature until soft enough to roll.
Store cookies in airtight containers, up to 1 week.