There are a lot of versions of this on the web; this is how I adapted a couple.
115g unsalted butter, 150g white sugar creamed together
2 eggs beaten into creamed sugar mixture
140g allpurpose flour,1 tsp baking powder sifted together
Add flour to egg/sugar/butter mixture in several batches. Base line 9″ springform tin, grease sides. Spread sponge mix in base of tin.
8 med/large round purple plums, stoned and sliced into 8 pieces each. Arrange plum slices in concentric circles over cake mixture, sprinkle with 1 to 2 tbsp sugar mixed with cinnamon.
Bake at 350F (conv bake) about 30 min.