Preheat oven to 350 degrees. Generously butter bundt pan and dust with flour, knocking out excess.
Whisk together flour, baking soda, salt and baking powder in a bowl.
Pulse walnuts, cinnamon, 1/4 cup sugar, and a pinch of salt in a food processor until nuts are chopped. Add melted butter and pulse once to combine.
Beat together remaining 1 1/2 sticks butter and remaining sugar in a large bowl with an electric mixer at high speed until pale and fluffy, about 3 minutes in a standing mixer or 5 minutes with a handheld. Reduce speed to medium and add eggs, 1 at a time, beating well after each addition. Beat in sour cream and vanilla until smooth and fluffy, about 2 minutes more in a standing mixer or 4 minutes more with a handheld. Reduce speed to low and add flour mixture, scraping down side of bowl occasionally and mixing until just combined.
Sprinkle half of nut mixture evenly in bottom of a bundt pan. Spoon half of batter over nuts and smooth with a rubber spatula. Sprinkle batter in pan with remaining nut mixture and top with remaining batter, smoothing top. Bake in middle of oven until cake begins to pull away from side of pan and a tester comes out clean, 45 to 50 minutes.
Cool cake in pan on a rack 1 hour, then run a thin knife around outer and inner edges. Place rack on top of pan, then fllip to invert cake onto rack. Cool completely.