Grease and line with a 20x20cm square tin with baking paper.
Place butter, sugar, egg and vanilla in a bowl and stir until combined.
Sift plain flour, self raising flour and cocoa into bowl. Add the coconut and stir until mixture has combined.
Spread the mixture evenly into pan and press down with a spoon to flatten the surface. Bake for 25-30 minutes or until just firm.
To make the icing, sift the icing sugar and cocoa into bowl.
Add the butter and boiling water and stir together.
Spread the icing over the still warm slice and decorate with sprinkles.
Thermomix Method
Preheat oven to 160 degrees celsius (fan-forced). Grease and line a square baking tin with baking paper and set aside.
Place butter into the Thermomix bowl and melt for 3 minutes, 100 degrees, Speed 1. Allow to cool.
Add the sugar, egg and vanilla extract to the Thermomix bowl and mix on Speed 5, 20 seconds. Scrape down the sides of the bowl.
Sift the flour and cocoa into the bowl and add the coconut. Mix on Speed 2, 20 seconds or until mixture has combined (use the spatula to assist with mixing).
Spread mixture evenly into pan and smooth the surface with a spoon. Bake for 25-30 minutes or until just firm.
Place the icing sugar and cocoa powder into the Thermomix bowl and press Turbo 5 times to sift.
Add butter and boiling water and mix together on Speed 5, 15 seconds or until smooth (add a little more water if the icing is too thick).
Spread warm slice with icing and decorate with sprinkles.
Notes
RECIPE NOTES & TIPSEquipment - this is a super easy one bowl recipe. All you need is a bowl, a spoon, a baking tray and an oven. You can also prepare the base and icing in a Thermomix. Both methods are included above. Icing sugar - for thick and firm icing, use pure icing sugar. For a soft icing, use icing mixture. If you're planning to use the slice in lunch boxes, I recommend using pure icing sugar.Runny vs. thick icing - if you like your icing to be thick, simply add a little more icing sugar and a little less water. If you prefer a runnier style of icing, add a tiny bit more water (but not too much!).Storage - This slice can be stored in the fridge or in a cool place at room temperature for up to 5 days. Alternatively, it can be frozen for up to 3 months.