For the plums, place the plums and sugar into a pan over a medium heat. Cook for 3-4 minutes, until the plums have softened.
Place three quarters of the plum mixture into a 1.2 litre/2 pint pudding basin and allow to cool. Reserve the remaining plum mixture in the pan.
For the sponge, place the sugar and butter into a bowl and whisk until light and fluffy.
Add the eggs one at a time, beating well to combine each egg before adding the next.
Fold in the flour and the plum liqueur and mix to a smooth thick batter.
Spoon the sponge mixture into the pudding basin over the plums.
Cover the bowl with greaseproof paper and secure around the edge with kitchen string. Cover with a sheet of kitchen foil and roll up around the edges to completely cover the paper.
Place a heatproof saucer or small plate upside down in the bottom of a large pan with a lid.
Place the pudding basin onto the saucer, then pour water into the pan to come two-thirds of the way up the side of the bowl.
Cover with the lid and bring to the boil. Reduce the heat and steam for 1½ hours. Remove the basin from the pan.
For the custard, place the egg yolks and sugar into a bowl and whisk until well combined.
Place the milk, cream and cinnamon sticks into a separate pan and bring to the boil. Reduce the heat and simmer for one minute.
Pour the egg and milk mixture onto the egg mixture and whisk together well.
Return the custard to the pan and continue to heat slowly (do not boil), until the custard has thickened and coats the back of the spoon.
To serve, reheat the reserved plums in their pan until hot.
Remove the foil and baking parchment from the pudding. Place a large plate over the top of the pudding basin and turn over, tipping the pudding onto the plate. Cut out slices of the pudding and place on each plate. Spoon the hot plums alongside and pour the custard over the pudding.
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