Toast the hazelnuts for a couple of minutes. Only lightly toasted. Then, grind them finely.

In a bowl, whisk together flour, cacao powder, ground hazelnuts, and salt. Set aside.

In a large bowl, beat butter and sugar until light and fluffy (about 2 minutes). Add the egg yolk and vanilla. Mix until smooth.

Add dry ingredients and mix just until combined. Dough will be soft but not sticky.

Divide the dough into two, flatten into discs, wrap in plastic, and refrigerate at least 1 hour (or overnight).

Preheat oven to 350°F. Line two baking sheets with parchment paper.

Roll dough to about ⅛–¼ inch (3–5 mm) thick on a large piece of parchment paper or lightly floured surface.

For half the cookies, cut a small shape (heart, star, or circle) in the center. For the other half, cut flat rounds. These two will make the tops of your sandwich cookies.*If the weather is warm, place baking sheets in the freezer for 10 to 15 minutes before baking.

Place cookies on sheets and bake 8–10 minutes, until edges are lightly golden. Cool completely on wire racks.

Dust cut-out tops with powdered sugar.

Spread ~1 tsp of Nutella on each solid base cookie. Gently sandwich with a sugar-dusted top.