Preheat oven to 350*. Thoroughly grease and flour a 9 springform pan. Sift flour with baking powder, baking soda and salt; set aside. Beat butter, sugar, eggs and vanilla with electric mixer on high until light and fluffy (about 5 minutes).
Reduce mixer speed to low. Alternately add about one-fourth of the flour mixture and one-third of the sour cream, beginning and ending with flour and beating thoroughly after each addition. After all has been added, scrape down sides of bowl and beat 1 minute longer.
Pour batter into prepared springform pan; spread evenly with rubber spatula. Put pan on center rack in oven and bake for about 50 minutes until cake is golden brown and a cake tester inserted in center of cake comes out clean.
Meanwhile, make Crumb Topping: Put sugar, flour, butter and cinnamon ina small bowl. Toss lightly with a pastry blender or two forks until mixture is crumbly and the butter is in fine pieces. Set aside.
Wash plums; dry well. Cut fresh plums in half, remove pits and discard. If using canned plums, drain thoroughly, cut in half and remove pits. Remove cake from oven and sprinkle with crumb topping. Arrange plums on top and bake 10 minutes longer.
Remove cake from oven and place on wire cake rack to cool for 10 minutes. Loosen cake from sides of pan, using a small knife or long metal spatula. Remove pan sides and brush plums with melted jam. Serve warm.