Place 1 tablespoon butter in a 9-in. round baking pan. Put pan in a preheated 350° oven to melt. Remove from oven and sprinkle with 1/3 cup sugar Arrange plum slices in a circular pattern over sugar. Bake for 10 minutes. Meanwhile, cream the remaining butter with remaining sugar; beat in the egg. Combine flour, salt and baking powder. Add to the creamed mixture alternately with milk and vanilla, beating until smooth. Spread evenly over plums. Bake for 30 minutes or until cake tests done. Remove from the oven and immediately invert onto a large serving platter. Serve warm.