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steaming to turn the power off and leave the buns for another 5 minutes.
This is similar to the guide I had followed - place the buns above cold water, then bring to heat, and steam for 5 minutes, then leave over the water for another 5 minutes.
I think there are a couple of obvious things I did wrong - I forgot to prove a second time, and I had to use a jury rigged steamer (with a roasting tin, foil and swearing).
I now have a proper bamboo steamer though, so we'll see how that goes.
Google suggests that it's bleached, low gluten / protein flour - I guess sponge flour would be good, in that case?