Preheat the oven to 350°F (175°C).

Line two large baking sheets with parchment paper or a Silpat mat.

In a skillet over medium heat, toast the hazelnuts for about 5-7 minutes until fragrant, then pulse in a food processor until finely ground.

In a medium bowl, whisk together the ground hazelnuts, flour, and kosher salt.

In a stand mixer, beat the softened butter and granulated sugar on medium-high speed until light and fluffy, about 3 minutes. Add the vanilla extract.

Gradually mix in the dry hazelnut-flour mixture until just incorporated.

Transfer the dough onto plastic wrap, form into a ½-inch thick disk, wrap securely, and chill in the fridge for at least 1 hour.

Dust the working surface with flour and roll out the chilled dough to about ¼-inch thickness.

Cut out cookies with a 2½-inch fluted round cutter and place on the lined baking sheets.

Chill the cut-out cookies in the fridge for 30 minutes.

Bake for 9-11 minutes until just set and slightly golden around the edges. Allow to cool completely on a wire rack.

Once cooled, pipe Nutella onto half of the cookies and sandwich with the other half.

Dust assembled cookies with confectioners' sugar before serving.