For the crumb topping: Preheat the oven to 350 degrees F. Spray a 9-inch springform pan with nonstick cooking spray.
In a medium bowl, combine the walnuts, sugar and cinnamon. Set aside.
For the batter: In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until fluffy, 3 to 5 minutes. Add the eggs, one at a time, to the butter mixture, allowing each to be fully incorporated. Add the vanilla paste and sour cream and beat until smooth.
Combine the flour, baking powder, baking soda and salt in a large bowl. Add to the batter and mix until combined.
Spread the batter in the prepared springform pan and sprinkle with the crumb topping. Bake until a cake tester inserted in the center of the cake comes out clean, about 45 minutes. Place the cake, still in the pan, on a wire rack and let cool to room temperature. Remove from the pan, then slice and serve with a dollop of sour cream if desired.