Step 1Heat the oven to 350 degrees. Grease a 10-inch springform pan. Set it aside on a baking sheet (to catch any juices).
Step 2Use a mixer to beat the butter and sugar until fluffy, about 2 minutes. Add the eggs; mix until smooth, about 30 seconds. Mix in the sour cream and vanilla. Sift together the flour, baking powder, baking soda and salt. Add to the batter. Mix until smooth. Spread evenly in the prepared pan. Arrange the plum slices evenly over the surface. Scatter the raspberries evenly over the plums. Sprinkle the cinnamon-sugar over the fruit.
Step 3Bake until a toothpick inserted in the center comes out clean, about 55 to 65 minutes. The batter will cover some of the fruit as it bakes. Cool the cake at least 1 hour before serving. (This can be made a day ahead and kept at room temperature, covered. To serve, reheat in 300-degree oven until warm, about 12 minutes.) Cut into wedges with scoops of ice cream.