Preheat oven to 160'C fan forced. Grease a 3cm-deep, 20 x 30cm (base) lamington pan. Line with non-stick baking paper, leaving a 2cm overhang on all sides.
Beat the butter, sugar and vanilla in an electric mixer until light and creamy. Add the eggs, one at a time, beating well after each addition.
Sift half the flour and cocoa over the butter mixture and stir until combined. Mix through half the milk and repeat with the remaining flour, cocoa and milk. Spoon into the prepared tin and smooth the top. Bake for 30 minutes or until a skewer inserted into the centre comes out clean. Cool in pan for 10 minutes then turn onto a wire rack and cool completely.
To make the icing, sift the icing sugar and cocoa into a bowl, add the butter and boiling water and stir until smooth.
Cut cake into 15 pieces. Place the macadamia and coconut into a food processor and pulse to blend together, tip onto a dish. Use a fork and dip one piece of cake into the icing mixture, shake off any excess and toss in the macadamia and coconut. Place onto a wire rack over a baking tray. Repeat with remaining cake, icing, coconut and macadamias.