Preheat the oven to 350 degrees F. Finely grind the hazelnuts in a food processor. Spread on a baking sheet; bake until golden, 12 minutes.
Whisk the flour, cornstarch, 1/3 cup of the ground hazelnuts, the baking powder, baking soda and salt in a bowl; set aside. Beat the butter and granulated sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 3 minutes. Add the egg yolks, liqueur and vanilla; beat until incorporated. Reduce the mixer speed to low; add the flour mixture and beat until just combined. Turn onto a sheet of plastic wrap; shape into a disk. Wrap and refrigerate until firm, at least 1 hour.
Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Line 2 baking sheets with parchment paper. Dust the dough with flour and roll out into a 6-by-12-inch rectangle (1/4 inch thick) on a floured surface. Cut into 1 1/2-inch squares; arrange 1/2 inch apart on the baking sheets. Cut out circles from the middle of half of the squares using a small cutter. Refrigerate until firm, 15 minutes.
Bake, switching the pans halfway through, until the cookies are golden on the bottom, 12 to 14 minutes. Let cool 5 minutes, then transfer to racks to cool completely. Stir together the dulce de leche and the remaining ground hazelnuts. Sandwich between the plain and cut-out cookies. Dust with confectioners' sugar.