Generously grease a 1.2-litre pudding basin with butter and line the base with a circle of baking paper. Butter a large square of foil. If your marmalade is very firm, stir in some of the stem ginger syrup: what you want is a loose mixture.
Spoon the marmalade into the base of the pudding basin and set aside.
Using a fork or balloon whisk, whisk the flour, cocoa, baking powder and salt together in a bowl. In a mixing bowl or the bowl of a stand mixer, beat the butter and sugar together until pale and fluffy.
Gradually beat in the eggs, adding a little of the flour mixture if it starts to curdle. Stir in the flour mixture and enough of the milk to form a soft dropping consistency. Fold in the chopped stem ginger.
Spoon the mixture into the basin and smooth the top. Make 2 pleats in the centre of the prepared foil, place buttered- side down and secure with string around the rim.
Place an upturned saucer in a large pan and place the basin on top. Pour in enough boiling water to come one-quarter way up the side of the basin.
Cover with a lid and simmer for 1 hour 45 minutes, topping up with more boiling water if necessary. Carefully remove the basin from the pan, run a knife around the sides to loosen and invert onto a plate. Serve with custard or ice cream.