Directions

1. Preheat oven to 325 degrees. Place 3 tablespoons butter in an 8-inch round cake pan; melt in preheating oven, about 4 minutes. Sprinkle melted butter with 1/2 cup brown sugar. Arrange overlapping plum wedges in a circle around edge of pan; repeat in center, covering bottom of pan. Set aside.
2. In a large bowl, beat together remaining 6 tablespoons butter, remaining 1/2 cup sugar, and egg. Beat in sour cream until blended. Set aside.
3. In a medium bowl, mix together flour, baking soda, salt, pumpkin-pie spice, and walnuts. Stir flour mixture into sour-cream mixture until just combined (batter will be thick).
4. Spread batter evenly over plums. Bake until a toothpick inserted in center of cake comes out clean, about 50 minutes. Cool in pan 5 minutes. Run a knife around edge of pan; invert onto a serving plate. Serve warm or at room temperature, with whipped cream if desired.