Preheat oven to 325F/165C. Prepare 9-inch (23 cm) bundt pan by either spraying with nonstick cooking spray and coating with flour, or greasing with shortening and coating with flour. Tap out excess flour.
Prepare walnuts by toasting on a baking sheet for 10-12 minutes or until golden. Remove tray from oven and turn down the oven temperature to 300F/150C. Chop walnuts and mix together in a small bowl with brown sugar and cinnamon. Set aside.
In a large mixing bowl, or bowl of an electric mixer, combine sugar and butter. Cream by hand or with electric mixer until light and fluffy, and until the color lightens. Mix in the eggs one at a time, creaming thoroughly until mixture comes together and is not separated. Add the sour cream and vanilla and mix briefly until the mixture is light and creamy, scraping the sides of the bowl every so often.
In a medium bowl, combine the flour, baking soda, and salt. Add flour mixture to the creamed mixture gradually with the mixer on low. Mix gently until no streaks of flour remain.
Transfer one-third of the cake batter into the prepared bundt pan and spread evenly. Spoon half of the walnut mixture over the batter and smooth evenly. Cover with another third of the batter, and spread evenly. Follow with the second half of the walnut mixture. Spread the remaining batter over the top and smooth evenly.
Bake for 60-70 minutes, or until the top is golden and a toothpick or skewer comes out clean. (There may be moist crumbs and this is okay.) Allow to cool on wire rack for no more than 10-15 minutes or the nut mixture will stick to the sides of the pan.
Invert cake onto wire rack and release from pan. Allow to cool to at least room temperature before cutting.