Heat oven to 350 degrees and grease a 6-cup tube pan. Mix walnuts, brown sugar, and cinnamon to make the topping, and set aside. Sift flour with baking powder, baking soda, and salt until well combined, and set aside. Cream butter and sugar until mixture is light; then beat in eggs and vanilla. Add flour mixture in thirds, alternating with sour cream. Beat well after each addition. Spoon half the batter into pan and sprinkle with half the topping. Add remaining batter and sprinkle with rest of topping. Bake 40 minutes without opening oven, then test for doneness with a toothpick. If it doesn’t come out clean, bake another 5 to 10 minutes. Cool in pan 10 minutes; then invert pan onto a rack and cool completely.