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 evenly distributed. Set aside.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2–3 minutes.
04 - Add eggs one at a time, mixing thoroughly after each addition. Stir in vanilla extract until incorporated.
05 - Add flour mixture in three additions, alternating with sour cream. Begin and end with flour mixture. Mix until just combined, taking care not to overmix.
06 - Mix brown sugar and ground cinnamon in a small bowl until evenly blended.
07 - Spoon half the batter into prepared Bundt pan. Sprinkle with half the cinnamon mixture and half the nuts (if using). Repeat layers with remaining batter, cinnamon mixture, and nuts.
08 - Gently draw a knife through the batter in a swirling motion to create marbled patterns. Do not over-swirl.
09 - Bake for 45–50 minutes until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
10 - Let cake cool in pan for 15 minutes. Invert onto wire rack to cool completely. Dust with powdered sugar before serving if desired.