Preparation Preheat oven to 350°F. and butter and flour a 9-inch round or square baking pan at least 2 inches deep. Make streusel: In a food processor pulse together streusel ingredients until combined well and crumbly. Make cake batter: 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. 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.