Butter a 1.2 litre heatproof bowl.
Remove the skin from clementines and slice the flesh thickly.
Line the buttered bowl with clementine slices starting from the center and moving up the sides.
Whisk butter, sugar, eggs, flour, cocoa, baking powder, and salt until light and fluffy.
Spoon the batter into the lined bowl and cover with pleated greaseproof paper and foil secured with string.
Create a foil handle for the bowl and place it into a saucepan containing a saucer and water.
Simmer the pudding in the covered saucepan for 1.5 to 2 hours.
Heat caster sugar and vanilla in a pan until a light caramel forms.
Add liqueur to flambé, then stir in orange and clementine juices and cook for 3 minutes.
Whisk in butter until the sauce combines and thickens.
Remove the bowl from the pan and discard the coverings.
Invert the pudding onto a serving plate.
Pour the sauce over the sponge and serve with crème fraîche.