Heat the oven to 320°F/160°C and line a large baking sheet with parchment paper.

In a medium bowl, whisk together the ground almonds, ground hazelnuts, and sugar.

In the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites and salt on medium-high speed until stiff peaks form. Reduce speed to low and gradually fold in the nut mixture in small additions, mixing until a thick, even dough forms.

Using your hands or a small scoop, shape the dough into 1-inch/2.5 cm balls. Roll each ball in powdered sugar and place on the prepared baking sheet, leaving about 1 inch/2.5 cm of space between cookies. Top each with a halved hazelnut.

Bake for 13 to 15 minutes, until the cookies are lightly cracked and just golden around the edges. Let cool on the sheet for 5 minutes before transferring to a rack.