I made this cake for my plum-loving husband and my upsidedown cake-loving daughter, it was well-received!
For the fruit and caramel, I greased the pan (and a glass measure) with clarified butter, then made caramel in the microwave. For the sugar in the caramel I used 25% of the weight of the fruit and added a little water. Poured the caramel in the pan and when cool, arranged the plum slices on top.
For the cake, I used 30% of the Golden Almond Lemon wedding cake (6” + 9” batter), omitting the lemon oil and zest and adding a little almond extract instead. It made a wonderful base for the fruit, but I think next time I’ll tweak it and use 20-25% of the recipe instead of 30% to get the right cake:fruit ratio.
One other thing I did that worked out well was to grind the nuts/sugar in the food processor, then continue with the processor for the rest of the batter, using the method for the plum-blueberry upsidedown cake in RHC. It was very nice not to have to lug out and clean up after two appliances! The cake was fairly dense, but for this I didn’t mind.
I used cassis to flavor it the whipped cream, which was a wonderful flavor with the baked plums.