Pre-heat the oven to 150°C (300℉).
Line a large oven tray with grease-proof paper. Draw a large rectangle of 36cm x 27cm (14" x 10½") on the paper.
Blitz the hazelnuts until they form a fine, gravely texture.
Remove 25g (2 tablespoons) and put aside to use as decoration.
Stir in the brown sugar.
In a bowl, whisk the egg whites until peaks form.
Add the icing (powdered) sugar gradually.
Fold the brown sugar/hazelnut mixture into the egg whites.
Pour out onto the baking tray and push up to the edges of the rectangle you have drawn, smoothing so that it has a fairly flat surface.
Bake right at the bottom of the oven for 16 minutes until lovely and golden brown.
Remove from the oven and allow to cool completely.