01 - Preheat oven to 350°F. Grease and flour a Bundt pan thoroughly, ensuring all crevices are coated to prevent sticking.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl until well blended. Set aside for later use.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes. The mixture should appear pale and increased in volume.
04 - Add eggs one at a time, mixing thoroughly after each addition. Stir in vanilla extract until fully incorporated.
05 - Alternate adding the flour mixture and sour cream to the butter mixture in three additions, beginning and ending with the flour. Mix gently until just combined to avoid developing gluten, which can make the cake tough.
06 - Mix brown sugar and cinnamon in a small bowl until evenly combined.
07 - Spoon half the batter into the prepared Bundt pan. Sprinkle with half the cinnamon mixture and half the nuts if using. Repeat with remaining batter, cinnamon mixture, and nuts.
08 - Gently draw a knife through the batter in a circular motion to create a marbled swirl pattern. Do not overmix.
09 - Bake for 45-50 minutes until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
10 - Allow cake to cool in the pan for 15 minutes before inverting onto a wire rack. Cool completely, then dust with powdered sugar if desired before serving.