Step 1

Preheat oven to 350°F. and butter and flour a 9-inch round or square baking pan at least 2 inches deep.

Step 2

In a food processor pulse together streusel ingredients until combined well and crumbly.

Step 3

In a bowl with an electric mixer beat butter with sugar until light and fluffy and add eggs, 1 at a time, beating well after each addition, and vanilla. Sift in flour with baking powder and salt and beat until just combined.

Step 4

Spread cake batter in pan, smoothing top, and arrange plum slices over it in slightly overlapping concentric circles. Sprinkle streusel over plum slices and bake cake in middle of oven 1 hour, or until a tester comes out clean. Coffeecake may be made 1 week ahead: Cool cake completely in pan on a rack and freeze, wrapped well in plastic wrap and foil. Reheat cake, unwrapped but not thawed, in a preheated 350°F. oven until heated through, 35 to 40 minutes. Cool cake slightly on a rack and sift confectioners' sugar over it. Serve coffeecake warm or at room temperature.