01 - Preheat the oven to 350°F. Thoroughly grease and flour a Bundt pan to prevent sticking.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl. Set aside for later use.
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 thoroughly after each addition. Blend in vanilla extract.
05 - Add flour mixture in three portions, alternating with sour cream. Begin and end with flour mixture, mixing only until just combined to avoid tough texture.
06 - Combine brown sugar and cinnamon in a small bowl, mixing until evenly distributed.
07 - Spread half the batter into the prepared Bundt pan. Sprinkle with half the cinnamon mixture and half the nuts if using. Repeat layers with remaining ingredients.
08 - Gently draw a knife through the batter in a swirling motion to distribute cinnamon mixture and create marbled pattern.
09 - Bake for 45–50 minutes until a toothpick inserted into the center emerges clean.
10 - Let cake rest in pan for 15 minutes before carefully inverting onto wire rack to cool completely.
11 - Dust cooled cake generously with powdered sugar before slicing and serving.