Preheat oven to 350 degrees F. Butter and flour 9- to 10-cup Bundt pan. Tap out excess flour.
FOR FILLING: Using fork, mix flour, butter and cinnamon in small bowl until crumbly. Stir in walnuts and maple syrup. (This will be a liquid streusel.)
FOR CAKE: Whisk flour, baking powder, baking soda and salt in medium bowl to blend. Using electric stand mixer and paddle attachment, beat sugar and butter in large until fluffy. Beat in eggs, one at a time. Mix in vanilla. Beat in flour mixture in two additions, alternating with sour cream, just until blended.
Spoon half of batter into prepared pan. Spoon about two-thirds of maple filling over batter. Spread remaining batter over filling and smooth top. Dot with remaining filling. Bake until tester inserted near center of cake comes out clean, about 40 minutes. Transfer pan to rack and cool 15 minutes. Run knife around pan sides to loosen cake. Invert cake onto rack, then invert cake, streusel side up, onto platter.
DO-AHEAD TIP: Cake can be made up to 2 days ahead. Wrap in plastic and store at room temperature.