Grind the nuts very finely or crush with a rolling pin.
Sieve the flour onto a large baking board. Place the ground almonds, hazelnuts, caster sugar, salt, small flakes of cold butter and the egg yolks in the middle.
Bring all the mixture together with your hands and knead into a dough.
Wrap the dough in aluminium foil and leave to rest in the refrigerator for 2 hours.
Pre-heat the oven to 190°C.
Divide the dough into about 4 pieces and roll each piece into a log ½ to ¾ inch thick. Cut the log into pieces about 2 inches long.
Bend each piece into a crescent and taper the ends. Place on a baking sheet and bake in the middle of the oven for about 10 minutes until golden brown.
Mix together the orange rind and icing sugar. When the cookies are cool enough to handle, dredge each cookie in the sugar mix.
Tap off any excess and leave the cookies to cool on a wire rack.