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.
Add the cocoa powder, self-raising flour, caster sugar, milk, eggs and vanilla extract. Mix fo 10 seconds, Speed 4 (MC removed).
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).