• Preheat oven to 350°F. Grease and flour a 10-inch tube pan and set aside.

To Make The Cake Batter:

• In a mixing bowl, whisk together the flour, baking powder, baking soda, and salt.
• In a second, large mixing bowl or the bowl of a stand mixer with the paddle attachment, combine the eggs, sugar, butter, and vanilla extract. Beat on medium speed until lightened and slightly thickened. The mixture may look a little curdled, which is fine.
  
• Gently stir the dry ingredients into the wet. Mix just until no dry streaks remain. Avoid over-mixing.
  
• Fold in the sour cream until thoroughly combined. The batter should be thick but spreadable.
  

To Make The Streusel:

• In a third bowl, mix together the walnuts, brown sugar, granulated sugar, cinnamon, and vanilla until the sugar is evenly moistened and the nuts are coated.
  

To Assemble And Bake The Cake:

• Spoon half of the cake batter into the bottom of the prepared pan and smooth into an even layer with a spatula. Sprinkle evenly with 1/2 the streusel.
• Dollop the remaining batter over the streusel and gently spread it out. It does not need to be perfectly smooth. Top with the remaining streusel.
  
• Bake for 45–55 minutes, or until the top is deep golden, the edges are just starting to pull away from the pan, and a toothpick inserted near the center (midway between the outer edge and inner tube) comes out clean or with a few moist crumbs.
• Cool the cake in the pan on a wire rack for 15–30 minutes, until the pan is warm but not hot to the touch.
• Run a thin knife around the outer edge and the center tube, then invert to release. Cool completely on the rack, streusel side up, before slicing, about another 45 minutes. Serve at room temperature or cover and refrigerate.