Preheat oven to 160 degrees celsius (fan-forced). Grease a large oven baking dish (6 cup capacity) and set aside.
Sift the self-raising flour and cocoa by pressing Turbo 5-10 times. Set aside in a separate bowl.
Place butter into the Thermomix bowl and melt on 100 degrees, Speed 1, 2 minutes. Allow to cool slightly.
Add the milk, vanilla extract and egg and mix on Speed 4, 20 seconds.
Add the sifted self-raisingflour, cocoa powder and caster sugar and mix on Speed 4, 5 seconds.
Scrape down the sides of the bowl and mix for a further 10 seconds or until smooth.
Pour the mixture into the prepared dish.
Sprinkle the extra cocoa powder and caster sugar over the pudding mixture.
Carefully pour over the boiling water.
Bake for 30 minutes or until firm on the top (but liquid underneath - as this is the sauce).
Allow to cool slightly, then dust with icing sugar. Serve with ice-cream.