Bash the cardamom pods in a pestle and mortar, remove the seeds and discard the pods. Grind the seeds to a powder, then place into a bowl.
Split the vanilla pod lengthways, scrape out the seeds and add to the bowl. Mix in the sugar, coconut, egg white, all the orange zest and half the juice.
Halve, destone and place the plums cut-side up on a baking tray.
Pile the coconut mixture into the holes.
Bake for 15 to 18 minutes, till the crumble is golden and the fruit is soft. Serve with vanilla ice cream.