Combine the granulated and dark brown sugars, walnuts, flour, vanilla, and cinnamon in a medium bowl. Add the butter and stir to make moist, coarse crumbs; set aside.
Preheat the oven to 350 degrees F.
Line the bottom of an 8-inch square cake pan with parchment paper; butter the paper and the sides of the pan.
Sift the flour, baking soda, pie spice and salt into a medium bowl. Beat the butter and granulated sugar in a large bowl with an electric mixer on medium speed until light and fluffy, about 5 minutes. Add the eggs 1 at a time, beating well after each addition; beat in the vanilla. Add the flour mixture in 3 parts, alternating with the sour cream in 2 parts, beginning and ending with the flour and mixing well between additions.
Spread 2/3 of the batter in the prepared pan and sprinkle with half the crumbs. Top with the remaining batter and crumbs. Bake until a toothpick inserted in the center of the cake comes out clean and the top has browned, about 1 hour and 10 minutes.
Cool the cake in the pan on a wire rack for 20 minutes. Invert out of the pan and then turn crumb-side up to cool completely.
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When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)