For best results, bake your cake the day before you want to make the lamingtons. Cut the cake into 21 squares.
To make the icing, sift your sugar and cocoa into a bowl. Mix in remaining ingredients except coconut and stir until smooth and shiny, using a little more water if necessary to reach the right consistency.
Use immediately, keeping the bowl over hot water. Add a little more boiling water if the icing begins to thicken as you work. Dip the squares of the cake into the icing. A carving fork or skewer is useful. Allow to drop for a minute and then roll in coconut. Leave on a wire rack to dry.