In a large bowl, cream butter and 1/2 cup sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, baking powder and salt; add to creamed mixture and mix well., Transfer to a greased 9-in. square baking dish. Top with plums. Combine cinnamon and remaining sugar; sprinkle over plums., For topping, in a small bowl, combine flour and sugar; cut in butter until mixture resembles coarse crumbs. Stir in walnuts if desired. Sprinkle over top., Bake at 350° for 50-55 minutes or until topping is golden brown and plums are tender. Cool on a wire rack.