Preheat to 325°F Line two baking sheets with Silpat or parchment paper.
Pulse nuts and sugar in a food processor until finely ground. Scrape into a large bowl.
Beat egg whites and salt in another large bowl with an electric mixer on medium-high speed until stiff peaks form.
Using a silicone spatula, fold the egg whites into the nut mixture. Add vanilla and gently but thoroughly mix until combined.
Transfer the mixture to a disposable piping bag and snip off the tip. Pipe the batter on the Silpat, making sure to keep a uniform size under 1 inch in diameter, as well as maintaining a fingers distance between cookies.
Tap the baking sheet a few times on the counter, this will help spread the cookie and give a flat cookie with a good snap.
Sprinkle the cookie batter with the sliced hazelnuts.
Place the baking sheet in the centre of the oven and bake until golden brown, 5-7 minutes. Switch the pans back to front. Keep an eye on it; it goes from golden brown to brown very quickly, given the tiny size.
Let cool on the baking sheets for 5 minutes. Gently transfer the butterless cookies to a wire rack to cool completely.
Store in an airtight container for upto a week. But I bet it won't last that long.