Gather all ingredients. Preheat the oven to 350 degrees F (175 degrees C). Grease a 10-inch springform pan.
Combine sugar and butter in a large bowl and beat with an electric mixer until light and fluffy. Add eggs, 1 at a time, and beat well after each addition.
Combine flour, baking powder, and salt in a separate bowl and sift on top of the butter mixture; mix to combine.
Transfer batter into the prepared springform pan and smooth out.
Set plums inside batter, cut-side up, and sprinkle evenly with cinnamon-sugar.
Bake in the preheated oven on the lowest rack until a toothpick inserted into the center comes out clean, 40 to 50 minutes.
Serve warm or cool to room temperature.
