1.Beat butter and sugar in small bowl with electric mixer until light and fluffy. Beat in treacle and egg yolk. Transfer mixture to a large bowl; stir in sifted dry ingredients. On a lightly-floured surface, knead dough until smooth. Cover and refrigerate 1 hour.
2.Preheat oven to 180°C (160°C fan-forced).
3.Roll dough between sheets of baking paper until 4mm thick. Cut shapes from dough using Christmas cutters. Make a small hole in the top of each biscuit for threading through ribbon, if desired. Place shapes 3cm apart on greased oven trays.
4.Bake, uncovered, about 10 minutes or until browned lightly (time depends on the size of the shapes). Transfer shapes to wire racks to cool.
5.To make royal icing, beat egg whites in a small bowl with electric mixer until soft peaks form. Gradually add sifted icing sugar, beat well between additions. Divide icing into several small bowls, tint each bowl of icing with colourings, as desired. Keep icing tightly covered with plastic wrap at all times to prevent it from drying out.
6.Spread or pipe royal icing onto cold biscuits; decorate with cachous.
For bold icing colours, we used powdered food colourings, available from cake decorating shops and some health food stores. Mixing the powder with a small amount of hot water, before stirring into the icing, gives a better result; liquid colourings can be used for pale colours.