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.