Butter your heat-proof Pyrex mixing bowl. Make sure you have adequate boiling water in a pan (or a conventional steamer) on the stovetop to steam the sponge.
Put the flour and cocoa powder into a processor and blitz to get rid of any lumps.
Add all the remaining sponge ingredients to the processor and blitz, for longer this time, to mix. Take the lid off, scrape it down, then put the lid back on for 3 more long pulses. Scrape the chocolate batter into the prepared bowl, smooth it down (the batter will come only halfway up the bowl) and wrap the bowl, first, in parchment paper, then in a layer of aluminum foil, followed by a layer of plastic wrap and a second layer of foil, so no water could possibly get inches Stem in the boiling water in the pan or steamer for 1½ hours (by which time the sponge will have risen to about 1½ inches below the lip). To cook it for longer will do no harm.
To make the sauce, which can easily be done before you eat, and reheated just before you serve the pudding, put all the sauce ingredients into a saucepan and place over a gentle heat to melt, stirring every now and again and then whisking, off the heat, at the very end, to combine smoothly.
When the sponge is ready, remove it carefully from the pan or steamer without burning yourself, then unwrap from its foil and plastic wrap casing. Put a plate, or a stand, with a slight lip, on top, flip both upside down, so plate and sponge are the right way up, and wiggle off the bowl.
Pour some hot sauce over the sponge, so that it just covers the top and falls in glossy, licking drips down the side, and pour the rest of the sauce into a jug or bowl to be served with a spoon.
Make Ahead Tips:
Measure all the sponge ingredients the day before, ready to whiz together.
Make the chocolate sauce up to 1 week ahead, then cool and chill until needed, reheating gently on the stovetop or in the microwave.
Freeze Ahead Tip:
Make and freeze the chocolate sauce for up to 3 months, and reheat as above.