Heat the oven to 350 degrees F and grease a 6-cup tube pan. Mix walnuts, brown sugar, and cinnamon to make topping and set aside. Stir or sift flour with baking powder, baking soda, and salt until dry ingredients are well combined.
Cream the butter with the sugar until the mixture is light, then beat in the eggs and vanilla. Add the flour mixture in alternate thirds with the sour cream, beating well after each addition. Spoon half the batter into the pan and sprinkle with half the topping, then add remaining batter and sprinkle on the rest of the topping. Bake for 40 minutes without opening the oven, then test for doneness (a toothpick should come out clean). It may need another 5 to 10 minutes of baking.