Preheat the oven to 180°C (160°C fan-forced).
Halve the plums; remove stones. Slice the plums; place in a large bowl. Add the caster sugar; toss until the plums are well coated in the sugar. Stand for 10 minutes.
Meanwhile, combine brown sugar, flour and coconut in a bowl; rub in the butter.
Stir the plums again to ensure they are well coated, then spoon into the base of six (2/3 cup/160ml) ovenproof dishes or a 1-litre (4-cup) ovenproof dish. Top the plums with the crumble mixture, place dishes on an oven tray. Bake for about 30 minutes or until golden and the juices are bubbling over the side.
Serve warm with whipped cream, ice-cream or custard, if desired.
Suitable to freeze. Not suitable to microwave.Note