Preheat the oven to 350°F. Lightly grease an 8 or 9 square or round pan.

Beat together the butter, sugar, baking powder, and salt until smooth.

Add the eggs, beating until thoroughly combined. Stir in the vanilla.

Add the flour, mixing to a smooth, stiff batter.

Spread the batter in the pan. In the larger-sized pans, the batter will be spread fairly thin; thats OK.

Spread the berries or plums on top. If you use plums, place them with their skins up. Sprinkle with the cinnamon-sugar.

Bake the cake for about 35 to 40 minutes, until 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.