01 - Preheat oven to 350°F. Grease and flour a Bundt pan thoroughly, ensuring all crevices are coated to prevent sticking.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt until well blended. Set aside.
03 - In a large bowl, beat softened butter and granulated sugar with electric mixer until light and fluffy, approximately 2-3 minutes.
04 - Beat in eggs one at a time, mixing thoroughly after each addition. Stir in vanilla extract until fully incorporated.
05 - Add flour mixture to wet ingredients in three additions, alternating with sour cream. Begin and end with flour mixture, mixing just until combined. Avoid overmixing.
06 - In a small bowl, mix brown sugar and ground cinnamon until evenly combined.
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 marble patterns, being careful not to disturb the pan's coating.
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. Carefully invert onto wire rack and cool completely before serving.
11 - Once completely cooled, dust generously with powdered sugar using a sieve for even distribution.