Preheat the oven to 190 degrees C. Lightly grease a 1.3 litre ovenproof dish, about 6cm deep.
Put the plums in a saucepan with 2 tablespoons of water. Simmer gently for about 5 minutes until the plums soften a little. Using a slotted spoon, spoon into the ovenproof dish with 2 tablespoons of the cooking juice.
Sift the flour and baking powder into a bowl and add the butter, sugar, almond meal, eggs and milk. Beat well for about 2 minutes until thoroughly blended.
Spoon the sponge mixture over the plums and smooth it level with the back of the spoon. Sprinkle evenly with the demerara or raw sugar.
Bake for 35–40 minutes, or until the sponge is well-risen and golden brown and will spring back when lightly pressed with your finger. Serve warm.