Preheat the oven to 350°F (175°C). Grease a Bundt pan thoroughly with butter or baking spray and dust lightly with flour.

Whisk together the flour, baking soda, and baking powder in a medium bowl. Set aside.

In a stand mixer or with a hand mixer, beat the softened unsalted butter and granulated sugar until pale and light, about 3 to 5 minutes.

Add the eggs one at a time, mixing briefly after each addition. Stir in the vanilla extract.

Add the dry ingredients and sour cream to the creamed mixture in three additions, alternating and starting and ending with dry ingredients. Mix until just combined. Fold in the walnuts if using.

Spoon half the batter into the prepared Bundt pan and spread it evenly. Sprinkle the cinnamon-sugar mixture over the batter.

Add the remaining batter on top and use a thin knife or skewer to swirl through the batter gently.

Bake for 45 to 50 minutes, or until a toothpick comes out clean or with a few moist crumbs. Rotate the pan halfway through if desired.

Let the cake cool in the pan on a wire rack for 15 to 20 minutes, then invert onto the rack to cool completely before slicing.