How to Make It
Preheat oven to 400°.
Lightly spoon flour into dry measuring cups; level with a knife.
Place 1/3 cup flour, 1/2 cup sugar, and ground cinnamon in a food processor, and pulse the mixture 2 to 3 times. With processor on, slowly add the oil and corn syrup through food chute, processing until the mixture resembles coarse meal. Remove the streusel mixture from the food processor, and set aside.
Place 1 1/3 cups flour, 1/3 cup sugar, baking powder, and salt in a food processor, and pulse mixture 2 to 3 times. Add the margarine, and process until mixture resembles coarse meal. Place margarine mixture in a large bowl.
Combine yogurt, water, vanilla, rind, and egg, and stir with a whisk. Stir yogurt mixture into margarine mixture in a large bowl until blended. Spoon batter into a 9-inch round cake pan coated with cooking spray. Sprinkle half of streusel mixture evenly over batter. Top with plums, arranging in a circular pattern. Sprinkle the remaining streusel mixture evenly over the plums.
Bake kuchen at 400° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool on a wire rack.