Mix the butter, icing sugar, egg and salt in a bowl using a wooden spoon. Mix the flour, cocoa and hazelnuts, add to the bowl, combine to form a dough. Flatten the dough a little, cover and chill for approx. 1 hr.
Between two sheets of baking paper, roll out the dough in batches to approx. 4 mm thick. Cut out approx. 45 equal-sized triangles, place on two trays lined with baking paper, chill for approx. 15 mins.
Approx. 10 mins. per tray in the centre of an oven preheated to 180°C. Remove from the oven, leave the cookies to cool on a rack.
To decorate the cookies
Melt the bags of cake icing in a bain-marie. Using a brush, coat ⅔ of the cookies with the dark icing and the remainder of the cookies with the white icing. Decorate half of the dark cookies as reindeer using salted pretzels, sugar eyes and Smarties. Turn the other half of the dark cookies into Christmas trees using the remainder of the white icing and the sugar pearls. Shape the marzipan carrots into 15 thin rolls, place on the white cookies as noses, draw on the eyes using writing icing.