Make the streusel topping. Put the flour and sugar in a small bowl. Add the butter and rub in with your fingertips until the mixture resembles fine breadcrumbs. Stir in the flaked almonds.
Make the cake. Beat the butter and sugar together in a mixing bowl until pale and creamy. Gradually mix in alternate spoonfuls of beaten egg and flour until all has been added. Stir in the baking powder, ground almonds and almond essence.
Spoon the cake mixture into a 23 cm (9 inch) spring-form tin lined with nonstick baking paper over base and sides, and spread the surface level. Arrange the sliced plums randomly over the top then sprinkle with the streusel topping.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 1 hour–1 hour 10 minutes, or until a skewer inserted into the centre comes out clean, covering the cake loosely with foil halfway during cooking if the top appears to be browning too quickly.
Leave to cool in the tin for 15 minutes then remove the tin and leave the cake to cool completely. When ready to serve, remove the lining paper and transfer to a serving plate. Dust with sifted icing sugar. Cut into wedges and serve plain or serve while still warm as a pudding with a spoonful of whipped cream or vanilla ice cream. Eat within 2 days.