Preheat the oven to 350°F. Sift flour, baking powder, and salt; set aside.
In an electric mixer, cream butter and the ¾ cup sugar. Add eggs and continue to beat until fluffy. Add the flour mixture and mix until just combined.
Spread the cake batter in a 9-inch greased springform pan. Cover the top with the plums halves, skin side up.
Mix the cinnamon with the remaining 2 tablespoons of sugar and sprinkle over the top.
Bake for 40 to 50 minutes, or until the center tests clean with a toothpick. Remove from the oven and let cool to room temperature, or serve while still warm.
