Preheat oven to 350 F. Lightly grease an 8″ or 9″ springform pan.
Cream the butter & sugar until light and fluffy. Scrape the bowl. Add the eggs; beat until smooth and light. Scrape the bowl again. In a small bowl, combine flour, baking powder, & salt; add to creamed mixture & mix JUST until fully combined.
Pour the batter into the cake pan. Arrange the plums, on top of the cake. Press plums down into batter. Drizzle with lemon juice & sprinkle with a Tbsp of sugar & 1 tsp cinnamon.
Bake for 50 – 60 minutes, depending on the size of the cake, or until a cake tester comes out clean, and the plums are juicy and bubbling.
Let cool before releasing the cake from the springform pan.