Arrange half the plums randomly over the base of a buttered and base-lined 20 cm (8 inch) springform tin.
Mix together the ricotta, 4 tablespoons of the sweetener, the egg yolks and almond essence in a bowl until smooth.
Whisk the egg whites in a second bowl until stiff, moist peaks form. Fold into the ricotta mixture, then spoon over the plums. Sprinkle the top with the flaked almonds and bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 30–35 minutes until the cake is well risen, golden brown and the centre is just set. Check after 20 minutes and cover the top loosely with foil if the almonds seem to be browning too quickly.
Turn off the oven and leave the cake to cool for 15 minutes with the door slightly ajar. Cool, then chill well in the refrigerator.
Meanwhile, cook the remaining plums with 2 tablespoons of water in a covered saucepan for 5 minutes until soft. Purée until smooth, mix in the remaining sweetener, if needed, then pour into a small jug.
Remove the tin and lining paper and transfer the cake to a serving plate. Dust the top with the icing sugar and serve, cut into wedges, with the sauce.