Queensland State Archives posted a photo:
Icing the Christmas Cake.
Before making the icing see that the top surface of the cake is quite flat. With a sharp knife trim it and grate off any blackened parts. It is difficult to give the exact proportion of icing, as some people like a thick layer and others a thin line. As you spread the icing over the cake, dip the knife constantly into a jug filled with hot water, which makes either almond paste or sugar icing work nicely.
Description source: The Telegraph, 18 December 1912
Image source: Queensland State Archives Item ID 1640281