Grease and line a 20cm sheet pan and set aside. Preheat oven to 170 degrees celsius (fan-forced).
Sift the cocoa powder and self-raising flour into a bowl.
Add the caster sugar, milk, eggs, vanilla extract and melted butter. Beat for 2-3 minutes or until smooth and creamy.
Pour into the prepared tin and bake for 25-30 minutes or until a skewer inserted into the middle comes out with a few crumbs on it. Set aside to cool before turning out onto a wire rack.
To prepare the icing, sift the icing sugar and cocoa powder into a bowl. Add the hot water and mix until smooth and slightly runny. If your icing is too thick, add a little more water.
Working quickly, spread the icing over the cake (use a hot knife to help spread the icing).
Decorate with sprinkles (optional).
Thermomix Method
Grease and line a 20cm sheet pan and set aside. Preheat oven to 170 degrees celsius (fan-forced).
Melt the butter for 2 minutes, Speed 1, 100 degrees.
Scrape down the sides of the bowl and mix for a further 20 seconds, Speed 4 (MC removed).
Pour into the prepared tin and bake for 25-30 minutes or until a skewer inserted into the middle comes out with a few crumbs on it. Set aside to cool before turning out onto a wire rack.
To prepare the icing, place the icing sugar and cocoa powder into the TM bowl. Press Turbo 5-10 times to sift.
Add the hot water and mix on Speed 6 until smooth and slightly runny (you'll need to scrape down the sides of the bowl partway through mixing). If your icing is too thick, add a little more water. Working quickly, spread the icing over the cake (use a hot knife to help spread the icing).
Decorate with sprinkles (optional).
Notes
RECIPE NOTES & TIPS
Don't skip sifting - sifting the self-raising flour, cocoa and icing sugar removes lumps and gives you a smoother batter and icing.
Room temperature ingredients - if your eggs and milk are cold, the batter can be heavier and may not rise as well.
Check for doneness early - start checking the cake around the 22-minute mark; every oven is different and chocolate cake can dry out quickly if over-baked.
Cool before icing - make sure the cake is completely cool before adding the icing, otherwise it will melt and run off the sides.
Adjust icing consistency - too thick? Add a tiny splash of hot water. Too thin? Add a little more sifted icing sugar.
Storing the cake - store the iced cake in an airtight container at room temperature for 3-4 days (if your kitchen is very warm, you can pop it in the fridge).
Freezing (un-iced) - wrap the completely cooled, un-iced cake well and freeze for up to 3 months. Thaw at room temperature, then ice and decorate just before serving.
Freezing (iced) - you can freeze the iced cake in slices, but for the prettiest finish I recommend freezing it un-iced and adding fresh icing once thawed.