Allow butter and eggs to stand at room temperature for 30 minutes. Meanwhile, grease and lightly flour a 10-inch fluted tube pan; set aside. In a medium bowl, stir together flour, baking powder, baking soda and salt; set aside.
In a large mixing bowl, beat butter with an electric mixer on medium speed for 30 seconds. Add sugar and vanilla; beat till combined. Add eggs 1 at a time, beating on low to medium speed 1 minute after each addition and scraping bowl frequently. Alternately add flour mixture and sour cream to butter mixture, beating on low speed after each addition just till combined.
In a small bowl, combine walnuts, sugar and cinnamon. Spoon half of the batter into the prepared pan. Sprinkle half of the walnut mixture over the top of the batter. Spoon the remaining batter over the walnut mixture. Top with the remaining walnut mixture.
Bake in a 350 degree F oven about 40 minutes or till wooden toothpick inserted near center comes out clean. Cool in pan for 10 minutes on a wire rack. Invert onto serving plate; serve warm or cool completely before serving. Makes 10 servings.