1) Preheat the oven to 350°F. Lightly grease an 8" or 9" square pan, or 9" or 10" round pan.
2) Beat together the butter, sugar, baking powder, and salt till smooth.
3) Add the eggs, beating till thoroughly combined. The mixture may look somewhat coagulated; that's OK. Stir in the vanilla and optional flavor.
4) Add the flour, mixing to make a smooth, stiff batter.
5) Spread the batter in the pan. In the larger-sized pans, the batter will be spread fairly thin; that's OK.
6) Spread the berries or plums on top. If you use plums, place them with their skins up. Sprinkle with the cinnamon-sugar.
7) Bake the cake for about 35 to 40 minutes, till a cake tester inserted into the center comes out clean, and the cake showing around the berries is a light golden brown. Remove it from the oven, and serve warm, or at room temperature.
8) Yield: one cake, about 10 servings.