Toast the hazelnuts in a pan just until golden brown, then remove from the heat and allow to cool. Once cold, chop and grind the hazelnuts into a coarse flour.
Whip the egg whites with sugar and, once white and shiny, add hazelnuts, flour, baking powder and lemon zest. Knead to form a compact ball.
Preheat the oven to 180°C.
Shape the dough into loaves and cut the loaves into rectangular cookies.
Place the cookies on a baking tray lined with parchment paper and bake for about 12-15 minutes, until they are golden brown. Let them cool completely before removing from the parchment paper.
Keep the cookies in a tin box or in a jar.