Step 1

Preheat the oven to 325°F (165°C).

Step 2

Toast the walnuts on a rimmed baking sheet until they’re a deep golden brown, 10 to 15 minutes. Transfer the walnuts to a plate to cool. Turn the oven down to 300°F (150°C).

Step 3

Finely chop the cooled toasted walnuts. In a small bowl, mix together the walnuts, brown sugar, and cinnamon.

Step 4

Spray a 9-in (23-cm) Bundt pan with nonstick cooking spray and coat with flour. Tap out any excess flour.

Step 5

In a large bowl, combine the sugar and butter. Beat with the paddle attachment of an electric mixer on medium speed until the color lightens, about 3 minutes. Add the eggs, 1 at a time, beating thoroughly after each egg until its completely incorporated before adding the next. Stir in the sour cream and vanilla and mix well until light and creamy. Scrape the sides of the bowl to make sure all of the ingredients are evenly incorporated.

Step 6

Combine the flour, baking soda, and salt in a separate bowl. Gradually add the flour mixture to the butter mixture and mix by hand or with a mixer on low speed until smooth and homogeneous.

Step 7

Scoop 1/3 of the batter into the prepared pan. Sprinkle 1/2 the nut mixture evenly over the batter. Cover with another 1/2 the remaining batter, using a spoon or spatula to spread the batter evenly over the pan and to the edges after each addition. Sprinkle the remaining nut mixture evenly over the batter and cover with the remaining batter, again spreading it evenly over the nut mixture.

Step 8

Bake for 60 to 70 minutes or until a cake tester or skewer or uncooked strand of spaghetti comes out clean. Cool the cake in the pan on a wire cooling rack 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.

Step 9

Place the wire cooling rack on top of the Bundt pan and then invert the pan to release the cake. Cool to room temperature before slicing. (The cake keeps well at room temperature for at least 1 week if wrapped well. The texture will become denser over time. It also freezes nicely for up to 3 months.)