Grease the slow cooker bowl thoroughly with the 20g/1oz butter.
In a large bowl, make the pudding mix. Mix the melted butter, flour, sugar, cocoa, baking powder, eggs, milk and orange extract together until smooth. Put into the greased slow cooker bowl and even out the top. Add most of chocolate orange segments evenly on top, pressing them into the mixture (keep a few segments back for decoration at the end).
Mix the sugar and cocoa powder for the topping in a small bowl, then sprinkle it evenly over the top of the pudding mix.
Carefully pour over the boiling water (it will all look a bit odd at the moment, but keep the faith!) Put the lid on the slow cooker and cook on HIGH for 1-2 hours until the pudding is firm in the middle.
Take off the lid and sit your reserved pieces of chocolate orange on top to decorate, put the lid back on and cook for a further 30 minutes, then serve.
Keep leftovers in the fridge and reheat in the microwave (or serve cold) for pudding the next day.
To make without a slow cooker:
Make as above in a shallow baking dish (about 27x20cm) and baking at 160℃/320F fan, 180℃/350F or Gas Mark 4, covered in foil, for 30-40 minutes.
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