Preheat oven to 180°C and lightly grease a 26 cm springform tin.
To make the topping, melt butter and stir in sugar and cinnamon, then allow to cool a little. Whisk eggs well and stir into cooled butter mixture.
Cream butter and sugar until light and fluffy, then mix in flours and salt. Mix eggs with milk, then add to flour mixture and mix to make a softish dough (it should drop easily from the spoon). Spoon batter into prepared tin (it should not fill more than a quarter of the depth, as the cake rises a great deal), smooth the top and sprinkle over ground almonds. Arrange plums on top, cut-side up, starting around outside edge and working towards the centre. Spoon topping over and around plums on the cake.
Bake for 50 minutes–1 hour until a fine skewer inserted comes out clean. Serve warm with cream or ice-cream. Any leftover cake can be warmed, wrapped in foil, in the oven at 180°C for 15 minutes.
