Make up the chocolate cake mixture. Grease a large steamer well and pour in the cake mixture. Cover with buttered foil and clamp steamer lid tightly over foil. Steam in a large covered saucepan (water level about 1" up side of steamer - check and replenish as needed during steaming) for about an hour (test with skewer). Turn out onto serving plate after it has cooled for about 10 minutes.
Put all the sauce ingredients into a small saucepan and bring to the boil slowly, stirring constantly. Simmer, uncovered, over very low heat for about 10 minutes, stirring occasionally, and watching that it doesn't boil over.
Serve warm pudding with chocolate sauce and cream/ice cream.