Butter a 1.7 litre pudding basin. To prepare the topping, melt the chocolate, then remove from the heat and stir in the sugar, cream and nuts. Spoon the mixture into the bottom of the pudding basin.
For the sponge, beat the butter and brown sugar together in a bowl until light and fluffy. Gradually add the eggs, beating all the time. Sift the flour and cocoa powder together over the mixture and fold in carefully.
Turn the sponge mixture into the basin.
If steaming:
Cover the top of the basin with a double thickness of greaseproof paper, making a pleat in the middle to allow for expansion. Secure under the rim with string.
Place the pudding basin in a steamer or stand on a trivet in a saucepan containing enough boiling water to come about halfway up the side. Cover tightly and steam for 1 ½ hours, topping up with moiré boiling water as necessary.
If microwaving:
Cover the top of the bowl with cling film, piercing the top.
Microwave on full power for 3 mins. If the sponge is not set, then cook again for another 45 sec.
Turn the pudding out onto a warmed plate and serve with chocolate sauce or poring cream.