Preheat the oven to 200°C /180°C fan assisted. Grease a 1.5 - 2L baking dish and set it aside.
Place the butter and milk in a large mixing bowl and melt the butter in the microwave in 20 seconds bursts. Alternatively, melt the butter on the stovetop and add the milk. Allow the mixture to cool before adding in the eggs and vanilla and beating until well combined.
Sift or whisk together the flour, cocoa powder, baking powder, salt and sugar.
Add the dry ingredients to the milk mixture and fold in until just mix through. Pour the batter into the prepared dish.
Whisk together the sauce's dry ingredients and sprinkle over the top of the sponge. Carefully pour the boiling water over the top of the dry ingredients, this can be done by allowing the boiling water to pour off the back of a spoon into the pudding. Bake for 25 - 30 minutes or until the middle of the pudding is just set. Allow to cool for 5 minutes and serve either as is or with a scoop of ice cream or some whipping cream.
Notes
Flour - plain, all-purpose or standard grade flour. If you use self-raising flour, remember to skip the baking powder. Brown sugar (light or dark or raw) is a must for the sauce, it adds a nice flavour. Cocoa powder should be preferably Dutch-processed. My favourite brand in New Zealand is Nestle. Serve this pudding warm, the longer the pudding sits, the more sauce is absorbed into the cake.