Preheat the slow cooker if necessary; see the manufacturer's instructions. Butter a 1.2 litre (2 pint) oval or round heatproof dish that will fit comfortably in your slow cooker pot and line the bottom with a piece of greaseproof paper. Arrange the plum halves, cut side down, in rings in the base of the dish.
Cream together the measured butter and sugar in a mixing bowl until light and fluffy. Gradually beat the eggs and ground almonds alternately into the mixture. Stir in the polenta, baking powder and orange rind and juice and beat until smooth.
Spoon the mixture over the plums and smooth with a knife. Cover the dish with buttered foil, then stand it on an upturned saucer or 2 individual flan rings in the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the dish. Cover and cook on high for 3–3½ hours or until the top of the cake is dry and springs back when pressed with a fingertip.
Remove the dish carefully from the slow cooker using a tea towel. Take off the foil and leave to cool slightly. Run a knife around the inside edge of the dish to loosen the cake and turn it out on to a serving plate. Remove the lining paper, sprinkle the top with toasted flaked almonds and dust with a little sifted icing sugar to decorate. Cut into wedges and serve warm or cold with spoonfuls of whipped cream, if liked.