Cook plums and 60g sugar in a pan over medium heat for 4 minutes until softened.
Place three-quarters of the plums into a 1.2 litre basin and reserve the rest.
Whisk butter and 175g sugar until light and fluffy.
Beat in eggs one at a time until combined.
Fold in flour and liqueur to create a smooth batter.
Spoon the batter into the basin over the plums.
Cover with greaseproof paper, secure with string, and wrap with foil.
Place an inverted saucer in a large pot and set the basin on top.
Add water to the pot until it reaches two-thirds up the side of the basin.
Cover, bring to a boil, and steam for 1.5 hours.
Whisk egg yolks and 40g sugar together for the custard.
Boil milk, cream, and cinnamon, then simmer for one minute.
Whisk the hot milk mixture into the egg mixture.
Heat the custard slowly until thickened enough to coat a spoon.
Reheat the reserved plums.
Invert the pudding onto a plate and serve with plums and custard.