Preheat oven to 350F. Butter a 9-inch springform pan or tube pan thoroughly, then dust with flour, tapping out excess. If using a springform, wrap the bottom with foil to catch any leaks.

In a small bowl, combine brown sugar, walnuts, and cinnamon. Use your fingers to break up any sugar clumps. Set aside.

In a large bowl with an electric mixer, beat butter and granulated sugar on medium-high speed until light and fluffy, about 3 minutes. Scrape down the bowl halfway through.

Beat in eggs one at a time, mixing well after each addition. Add vanilla and beat until combined. The mixture should look smooth and pale yellow.

In a separate bowl, whisk together flour, baking powder, baking soda, and salt. With mixer on low, add flour mixture in three additions alternating with sour cream in two additions, beginning and ending with flour. Mix just until no dry streaks remain - do not overmix.

Spread half the batter into prepared pan and smooth the top. Sprinkle filling evenly over batter, leaving a 1/2-inch border. Dollop remaining batter over filling and gently spread to cover, using an offset spatula or back of a spoon.

Bake until cake is golden brown and a toothpick inserted in the center comes out clean, 45 to 50 minutes. The top should spring back when lightly pressed. Cool in pan on a wire rack for 15 minutes, then remove springform sides or invert tube pan to release. Serve warm or at room temperature.