01 - Preheat oven to 350°F. Generously grease and flour a Bundt pan, ensuring all crevices are coated for easy release.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl until thoroughly combined.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2–3 minutes.
04 - Incorporate eggs one at a time, mixing well after each addition. Stir in vanilla extract until fully blended.
05 - Add flour mixture in three additions, alternating with sour cream and beginning and ending with flour. Mix until just combined, being careful not to overmix.
06 - Combine brown sugar and cinnamon in a small bowl, mixing until evenly distributed.
07 - Spread half the batter in the prepared pan. Sprinkle with half the cinnamon mixture and half the nuts if using. Repeat layers with remaining ingredients.
08 - Gently drag a knife through the batter in a circular motion to create a marbled swirl pattern.
09 - Bake for 45–50 minutes until a toothpick inserted into the center comes out clean.
10 - Let cake cool in the pan for 15 minutes, then carefully invert onto a wire rack to cool completely.
11 - Dust cooled cake generously with powdered sugar before serving.