To make the icing:
- Whisk the icing sugar and cocoa powder together in a medium bowl.
- Combine the margarine and boiling water in a jug and allow the margarine to melt completely.
- Pour the water and margarine into the icing sugar and cocoa powder then whisk until smooth.
To assemble the lamingtons:
- Pour desiccated coconut onto a shallow dish.
- Dip the bottom of the cake in the icing, then place on a fork. Use a large spoon to scoop icing over the top of the cake until it is completely covered.
- Allow excess icing to drip off, then cover in coconut and press gently to help it stick.
- Place on a cooling rack to set.