Preheat oven to 180 C / Gas 4.
Beat sugar and butter until fluffy with electric mixer. Mix flour and baking powder and sift into creamed mixture. Beat in eggs and salt. Mix everything well.
Pour batter into a greased 26cm round springform tin and smooth out. Top with the plums (cut side up) and sprinkle with 1-2 tablespoons sugar and cinnamon.
Bake in preheated oven on lowest shelf for 40-50 minutes or until a skewer comes out clean. Cool to room temperature or serve warm.