Preheat the oven to 180°C. Grease a 30 cm x 20 cm lamington tin and line with baking paper.
Cream the butter, sugar and vanilla until soft and fluffy. Add the eggs and continue to beat until combined.
Add sifted flour and salt to the creamed mixture. Add milk and stir gently until just combined.
Pour the mixture evenly in the tin and bake for 30–35 minutes. Cool on a wire rack, then cut into rectangles. You should get about 20.
To make the icing, sift the icing sugar and cocoa into a bowl. Add the boiling water, butter and vanilla, then stir until dissolved.
Spread the desiccated coconut onto a cutting board. Using two forks to hold the sponge, dip the rectangles one at a time into the chocolate icing then immediately roll in the coconut. Leave on a wire rack to set.