Start by making the pastry. Place both flours and ground hazelnuts plus a good pinch of salt in a large mixing bowl. Rub the cold butter into the flour with your fingertips until you achieve the texture of bread crumbs. Mix in brown sugar, egg yolk and ice cold water using a fork. Bring together with your hands, working it into one lump. Wrap in cling film and chill for 1 hour.