Preheat oven to 350°. In a small bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Beat in eggs. Combine flour and baking powder; add to the creamed mixture alternately with milk, beating well after each addition. Add lemon zest. Pour into a greased 10-in. springform pan. Arrange plums on top; gently press into batter. Sprinkle with brown sugar and cinnamon. , Place pan on a baking sheet. Bake until top is golden and a toothpick inserted in the center comes out clean, 40-50 minutes. Cool for 10 minutes. Run a knife around edge of pan; remove rim. Cool on a wire rack. If desired, dust with confectioners' sugar just before serving.