Preheat the oven to 180 degrees C. Line and grease a 23cm cake tin.
For the cake
Put the butter, caster sugar, self-raising flour, polenta, baking powder and cinnamon in a food processor and pulse to combine. Add the eggs, orange juice and zest and blend until smooth.
For the topping
Put the butter and sugar in a frying pan and simmer until the sugar is a caramel colour. Add the plums and simmer a minute more. Add the Marsala and walnuts and simmer one more minute.
Pour the fruit mixture into the lined tin. Spoon the batter evenly over the top, smoothing the surface. Bake 40-50 minutes or until a skewer inserted into the middle comes out clean. Let cool in the tin for 20 minutes before turning out onto a serving platter.