Preheat your oven to 350°F. Butter and flour a 10-inch springform or tube pan to ensure easy release.
In a large bowl, whisk together the melted butter, granulated sugar, eggs, sour cream, and vanilla extract. Mix until well combined.
In a separate bowl, stir together the flour, baking powder, and salt.
Add the dry ingredients to the wet ingredients. Stir just until combined; do not overmix.
In a small bowl, combine the finely chopped walnuts, dark brown sugar, and cinnamon.
Spread one-third of the batter into the bottom of the prepared pan and sprinkle with one-third of the walnut topping. Repeat this process two more times, finishing with the remaining topping spread evenly across the top.
Place in the center of the oven and bake for about 1 hour. The cake is done when a skewer inserted into the middle comes out clean.
Allow the cake to cool in the pan. Run a sharp knife along the sides to release it before opening the springform.