Preheat you oven to 180°C (fan-forced) and line an 18 x 28cm slice tin with baking paper, making sure to leave plenty hanging over the sides.
Place the butter (which has been cut into cubes) into your Thermomix bowl and cook for 3 mins, 60 degrees, speed 2 to melt.
Add the flour, caster sugar, desiccated coconut and cocoa to your Thermomix bowl and mix for 10 seconds on speed 4.
Scrape down the sides of your Thermomix bowl and mix for a further 10 seconds on speed 4.
Press mixture into your prepared tin and place in the oven to bake for 16 - 18 minutes.
Remove from oven and allow to cool slightly.
To make the icing, combine the icing sugar and cocoa, add hot water a tablespoon at a time until your icing is thick but still can be spread using a butter knife.
Spread icing over the warm slice and sprinkle the coconut over the top before placing in the fridge to chill. Once the slice has completely cooled, cut into small squares and store in an airtight container in your pantry.