Beat the butter and sugar together in a mixing bowl until pale and creamy. Stir in the egg yolks, cocoa powder and cinnamon then gradually mix in the flour to form a smooth soft dough. Chill for 15 minutes.
Roll out the dough between 2 sheets of nonstick baking paper until 5 mm (¼ inch) thick. Stamp out festive shapes using biscuit cutters about 7.5 cm (3 inches) in diameter. Transfer to greased baking sheets. Reknead the trimmings and continue rolling and stamping out until all the mixture has been used.
Make a small hole in each biscuit using the handle of a teaspoon then bake at 180°C (350°F), Gas Mark 4, for 10–12 minutes until lightly browned. Remake the hole in each biscuit then leave to cool on the baking sheets.
Mix the icing sugar and the egg white or water to a smooth thick icing. Spoon into a greaseproof paper piping bag, snip off the tip and pipe lines, dots and swirls to decorate the biscuits. Leave to harden then thread narrow ribbons through the holes and hang on the Christmas tree or on white painted twigs standing in a jug.