Step 1

Make the Cinnamon-Walnut Swirl: Preheat the oven to 325°F and set an oven rack in the middle position. Toast the walnuts on a rimmed baking sheet until they’re fragrant and golden brown, 10 to 15 minutes. Transfer the walnuts to a plate to cool. In a small bowl, mix together the walnuts, brown sugar, and cinnamon. Set aside.

Step 2

Reduce the oven temperature to 300°F. Spray a 9- or 10-in Bundt pan with nonstick cooking spray and dust with flour. Tap out any excess flour. Alternatively, spray the pan with a nonstick spray with flour in it, such as Baker's Joy or Pam with Flour.

Step 3

In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

Step 4

In the bowl of an electric mixer fitted with the paddle attachment or beaters, combine the sugar and butter. Beat on medium speed until pale and fluffy, about 3 minutes. Add the eggs, one at a time, beating well after each addition until the mixture is homogenous. On low speed, mix in the sour cream and vanilla. The batter will look a little curdled at this point; that's okay. Scrape down the sides and bottom of the bowl to make sure all of the ingredients are evenly incorporated.

Step 5

Gradually add the flour mixture and mix on low speed until the batter is smooth and homogeneous.

Step 6

Scoop 1/3 of the batter into the prepared pan. (It won't seem like enough, especially if you're using a 10-in Bundt -- that's okay. Just do your best to smooth it into an even layer with a spatula or back of a spoon.) Sprinkle half of the cinnamon-walnut mixture evenly over the batter. Cover with another 1/3 of the remaining batter, using a spoon or spatula to spread the batter evenly over the pan and to the edges. Sprinkle the remaining nut mixture evenly over the batter and cover with the remaining batter, spreading it evenly over the nut mixture.

Step 7

Bake for 65 to 75 minutes, until the cake is golden and a cake tester comes out clean. Cool the cake in the pan for 15 minutes. Do not let the cake cool in the pan for much longer than this or the brown sugar in the streusel might stick to the sides of the pan and make it difficult to release the cake. Place a wire rack on top of the Bundt pan and then invert the pan to release the cake. Let the cake cool completely before serving. The cake will keep well in an airtight container at room temperature for up to a week.

Step 8

Freezer-Friendly Instructions: The cake can be frozen for up to 3 months. After it is completely cooled, double-wrap it securely with aluminum foil or plastic freezer wrap, or place it in heavy-duty freezer bag. Thaw overnight on the countertop before serving.