Preheat the oven to 170C.
Cream the butter and sugar until light and fluffy. Add the eggs and vanilla, mixing through.
Fold through the flour, cinnamon and milk, stirring to form the batter.
Drain the plums, reserving half a cup of juice. Halve and remove the stones from the plums. Place them with the juice in an ovenproof dish with the sliced ginger about 20 x 20 cm.
Spoon the batter on to the plums in blobs. It will spread as it cooks. Place into the oven for 40 minutes until the sponge is cooked through.
Serve warm, dusted with icing sugar and ice cream or whipped cream to finish.