Preheat oven to 160 degrees celcius.
Grease a large oven baking dish and set aside (minimum of 6 cup capacity).
Melt the butter in the microwave and allow to cool slightly.
Add the milk, vanilla extract and egg and which together.
Sift the self-raising flour and cocoa powder into a bowl. Add the caster sugar and mix together.
Pour the liquid over the flour mixture and stir to combine.
Pour the mixture into the prepared dish.
Boil the water and set aside.
Sift the extra cocoa powder over the pudding mixture.
Sprinkle over the extra caster sugar.
Carefully pour over the boiling water.
Bake for 30 minutes or until firm on the top.
Allow to cool slightly, then dust with icing sugar.
Serve with vanilla ice-cream or custard.