Preheat oven to 170 degrees Celsius (fan-forced). Grease and line an 18cm x 28cm rectangular slice tin with baking paper and set aside.
Place the butter into the Thermomix bowl and melt for 3 minutes, 80 degrees, Speed 2.
Add the cocoa powder, self-raising flour, caster sugar, desiccated coconut and melted butter and mix for 10 seconds, Reverse, Speed 4. Scrape down the sides of the bowl and repeat for a further 10 seconds.
Press the mixture into the prepared baking tin and bake for 20 minutes or until slightly firm on top. Allow to cool.
Make the topping by placing the cocoa powder, desiccated coconut, icing sugar, sweetened condensed milk, softened butter and boiling water into a clean and dry Thermomix bowl and mix for 10 seconds, Reverse, Speed 4. Scrape down the sides of the bowl and repeat for a further 10 seconds.
Use a hot knife (dipped in hot water and then dried off) to spread the topping over the cooled slice.
Place into the fridge to set before cutting into slices.
Store in an airtight container at room temperature for up to 1 week or wrap well and freeze for up to 1 month.