Preheat the oven to 375 degrees F and butter a 10-inch round cake pan.
Cream together the butter, sugar, eggs, flour, almonds, milk, vanilla, boiling water and baking powder in a food processor, running it for 10 seconds. Run a spatula round the bowl and process for 5 seconds more. Pour into the prepared pan.
Scatter the prunes evenly on the batter. Spoon over walnut and sugar mixture. Arrange the apple slices on top of the walnuts. Bake for 45 minutes.
Sprinkle the surface with sugar and cinnamon. Dot with butter and bake for 20 to 25 minutes more, or until a skewer comes out clean. Cool in the tin.