Preheat the oven to 180 and grease a 22cm spring form tin.
Cream the butter and sugar until light and creamy. Add the eggs and sift in the flour and baking powder and mix to combine.
Spoon the batter into the spring form pan and smooth on top, covering with the plums. Sprinkle the plums with the lemon juice and the extra sugar.
Bake for 45 to 50 minutes, or until a cake tester inserted into the middle comes out cleanly. Cool in the tin for 10 minutes before removing.
The original recipe uses cinnamon. I have made it both with and without the cinnamon and find I prefer it without. If you choose to add the cinnamon the recipe suggests 1 tbsp. This cake is also suitable for freezing.