Preheat the oven to 177 C/ 350 F and line 3 regular baking sheets with dimensions 9" x 13", with parchment paper.
In a medium bowl, combine all dry ingredients, ground almonds, hazelnuts, salt, fine granulated sugar and flour (except powdered sugar), add milk to the bowl and stir well with a plastic spatula to make a nutty paste. Set aside.
125 g (1 cup) almonds in powder, 125 g (1 cup) hazelnuts in powder, 100 g ( 3/4 cup) all-purpose flour, 135 ml (1/2 cup) milk 2%, ½ teaspoon salt, 250 g (1 cup) fine granulated sugar
In a large bowl of standing mixer with whisk attachment, place 19 egg whites and whisk first at low speed. Once the egg whites start creating a foam (mousse), increase the speed to high and whisk until the egg whites almost reach the peaks. At this point, slow down the speed and start adding slowly powdered sugar. Once all the sugar is incorporated, whisk on high speed until you reach nice rich white peaks.
590 g egg whites, 100 g (1 cup) powdered sugar
Take a ⅓ of the nutty paste and add it to the whisked egg whites. Mix with the whisk attachment on very low speed. Continue adding slowly the other ⅓ of the nutty paste and continue mixing. Now add the last ⅓ of the nutty paste and mix it in on low speed, until everything is well combine.
Divide the batter in three equal portions and spread each portion of the batter over 3 baking sheets with dimensions 9" x 13", covered with parchment paper. Bake on 177 C / 355 F for 10–12 minutes, or until lightly golden brown on top. Insert a toothpick to make sure, it is fully baked. Allow the biscuits to cool down before removing parchment paper on the bottom.