Streusel: Mix all ingredients together, well. Batter: Gently mix plum halves with 1/4 cup sugar until fruit is evenly coated. Combine the sifted flour, baking powder, sugar and salt. Sift into mixing bowl. Cut shortening into dry ingredients with pastry blender. Add egg, beaten until thick and lemon colored. Combine with milk. Stir just until dry ingredients are moistened. Spread half of batter into buttered 9x9x2" baking dish. Sprinkle with half the streusel mixture. Distribute 16 plum halves evenly over the surface, spread remaining batter over the plum halves; cover with the remainder of the streusel mixture and top with the remaining plum halves. Bake in a 350 degree over until done, about 50 minutes. Yield 16 generous squares. Wrap any leftover coffee cake in foil to keep fresh for eating cold or heated the next day.