Into a bowl, sift together the flour and the baking powder, add the flour mixture to the butter mixture, and stir the batter until it is combined well. Turn the batter into a well-buttered 9-inch springform pan and arrange the plums, skin side down, in a decorative pattern on it.

In a small bowl, stir together the remaining 1/2 cup sugar and the cinnamon, sprinkle the sugar mixture over the plums and bake the cake in the middle of a preheated 350 degree oven for 45 to 50 minutes or until it is golden. Let the cake cool in the pan on a rack for 20 minutes and remove the side of the pan.