Butter a 1.2-litre pudding basin and put a circle of buttered baking parchment in the base.
Sift together the cocoa, flour, bicarb, baking powder and a pinch of salt. Beat the butter and sugar together in an electric mixer until pale and fluffy, then add the orange zest. Gradually add the eggs, a little at a time, beating well after each addition. Stir in the marmalade. Fold in the sifted ingredients followed by the dark chocolate chunks. Scrape the batter into the prepared pudding basin.
Put a piece of baking parchment on top of a sheet of foil (both should be large enough to cover the basin and overhang the edges). Fold a pleat along the middle, then place this, parchment side down, onto the pudding. The pleat should run across the centre. Tie to the basin firmly with string, using it to make a handle. Trim off the excess parchment and foil.
Put the pudding in a large saucepan on a trivet or upturned saucer. Add enough boiling water to come one-third of the way up the sides of the basin. Bring the water to the boil, turn down to a simmer, cover the pan and steam the pudding for 2 hrs-2 hrs 15 mins. Keep topping up with hot water.
Lift the pudding out of the pan, leave to sit for 10 mins, then run a knife between the basin and the edge of the pudding. Invert onto a plate – the pudding should slide out. Remove the parchment.
Whip the cream and icing sugar to soft peaks, then stir in the marmalade. Serve the pudding with a dollop of the cream and some marmalade on top.