01 - Preheat 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. Mix only until just combined to avoid tough texture.
06 - Combine brown sugar and ground cinnamon in a small bowl. Mix until evenly incorporated.
07 - Spread half the batter into 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 create marbled pattern. Do not overwork.
09 - Bake for 45-50 minutes until a toothpick inserted into the center emerges clean.
10 - Allow cake to rest in pan for 15 minutes before carefully inverting onto wire rack. Cool completely.
11 - Dust cooled cake generously with powdered sugar before slicing and serving.