Grease a bundt pan, and set your oven to preheat at 350°F.
Using a stand mixer fitted with the paddle attachment or an electric mixer and a large mixing bowl, cream butter and sugar together on high speed for about 1 minute.
Add eggs, sour cream, and vanilla extract. Mix on medium speed to combine.
Add baking powder, baking soda and flour. Mix on low speed, just until combined. Avoid over mixing, which will cause your cake to be dry.
Set the batter aside. Combine the cinnamon, walnuts, and sugar for the filling/topping together in a small bowl.
Add one-half of the batter to the prepared bundt pan. Cover the batter with one-half of the cinnamon walnut mixture, then pour remaining cake batter over the top.
Sprinkle remaining cinnamon walnut mixture over the top of the cake.
Bake in preheated oven for 44-55 minutes, or until a cake tester comes out clean when inserted in the center.Allow cake to cool in pan for 10-15 minutes before turning it out onto a wire rack to cool completely.
Notes
Grease the pan well. Be sure to get into the nooks and crannies. I find it easiest to melt butter and use a pastry brush to paint it on.
Avoid over mixing the batter.
Test the cake for doneness. To be sure that the cake is baked all the way through, insert a toothpick or knife into the cake. If the tester comes out dry or with a few moist crumbs on it, it's ready to come out of the oven!