Pre-heat oven to moderate (180 degrees celsius, fan-forced).
Grease a slice tin (approximately 28 x 18cm) and line with baking paper - leaving longer edges on two opposite sides so you can lift the slice from the tray once set.
Cream butter and sugar until a lighter, creamy colour.
Stir through coconut.
Stir through sifted flour and cocoa until well combined.
Pour the mixture into the prepared slice tin and press out evenly using your fingertips. Even out further using the back of a spoon.
Bake for 20 minutes.
Make the icing during the baking time as this slice is iced while hot.
Icing
Sift icing sugar and cocoa into a bowl.
Add coconut and mix to combine.
Stir through condensed milk.
Add enough of the boiling water to form a paste that will be runny enough to spread over the biscuit base.
Once biscuit base is removed from the oven, pour the icing over the base and spread out evenly with a butter knife.