Grease a 1.2 litre pudding basin with butter.
Mix flour, cocoa, sugar, suet, and grated orange zest in a large bowl.
Gradually stir in milk to form a soft, sticky dough.
Roll out one third of the dough to form a lid using the basin as a guide.
Roll out the remaining dough into a 30cm circle and line the basin.
Place half the butter, brown sugar, and chocolate into the lined basin.
Pierce the orange with a skewer and place it on top of the sugar mixture.
Add the remaining butter, sugar, and chocolate around the orange.
Dampen the pastry edges, apply the lid, and seal tightly.
Cover the basin with pleated parchment and foil, then secure with string.
Steam the pudding in a large pan of simmering water for 2.25 hours.
Invert the pudding onto a deep dish and serve immediately with cream.