Step 1
Heat oven to 350°F. On a half-sheet pan lined with parchment paper, spread the hazelnuts and roast until golden brown. About 10 minutes.
Step 2
Remove the hazelnuts from the oven and allow them to cool. When cool to the touch, coarsely chop them. Set aside.
Step 3
In a large bowl of an electric mixer, fitted with the paddle attachment, mix powdered sugar, cocoa powder, salt, and chopped hazelnuts. Mix at low speed until combined. Add egg whites and vanilla and mix at medium speed for about 4 minutes. The mixture will look slightly thickened.
Step 4
With a small ice cream scoop (2 ounce), scoop the batter into cookie sheets lined with parchment paper. Space the cookies about 2 inches apart. Sprinkle sea salt on top of the cookies. Put the cookies in the oven and immediately lower the temperature to 325°F. Bake for 15 minutes or until cookies look glossy and lightly cracked.
Step 5
Remove from oven. Slide cookies into a cooling rack and allow them to cool down.