1. Preheat oven to 425°.
2. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, 2 tablespoons granulated sugar, brown sugar, baking powder, 1/4 teaspoon salt, and cardamom in a medium bowl, stirring well with a whisk. Cut in 4 tablespoons butter with a pastry blender or two knives until mixture resembles coarse meal.
3. Combine milk, vanilla, and egg in a bowl, stirring with a whisk. Add milk mixture to flour mixture, and stir until just combined.
4. Spoon batter into a 9-inch round metal cake pan coated with cooking spray. Arrange plums in a circular pattern over batter.
5. Combine remaining 2/3 cup granulated sugar, remaining 1/8 teaspoon salt, lemon rind, and allspice in a small bowl, stirring well. Place remaining 3 tablespoons butter in a microwave-safe bowl. Microwave at HIGH 30 seconds or until butter melts. Stir into sugar mixture. Sprinkle plums evenly with sugar mixture. Bake at 425° for 35 minutes or until browned and bubbling. Cool in pan 1 hour on a wire rack. Cut into wedges.