Beat butter, sugar and treacle until light and creamy. Add egg, beat well.
Combine flour, spices and baking soda and stir into creamed mixture, mix until just combined.
Turn onto floured surface, knead 1 minute, adding a little extra flour if dough is sticky. Wrap dough in plastic wrap and refrigerate 30 minutes or until firm.
Preheat oven to 180°C.
Roll out dough onto lightly floured surface to 4mm thickness. Using Christmas cutters (we've used angels), cut shapes from dough.
Place shapes on baking trays, lined with baking paper, allowing room for the biscuits to spread. Bake 10-12 minutes. Allow to cool.
Beat egg white until frothy. Gradually add icing sugar, beating well after each addition. Continue until the icing is firm and holds its shape.
Divide mixture into portions and tint with colour as desired; mix well.
Pipe/spread icing onto biscuits decoratively.