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.
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. Stir in vanilla extract.
05 - Alternate adding flour mixture and sour cream in three portions, beginning and ending with flour. Mix until just combined, avoiding overmixing.
06 - Combine brown sugar and cinnamon in a small bowl, mixing until evenly distributed.
07 - Spread half the batter into prepared 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 patterns.
09 - Bake for 45-50 minutes until a toothpick inserted into the center emerges clean.
10 - Let cake cool in pan for 15 minutes, then invert onto wire rack to cool completely.
11 - Dust with powdered sugar before serving, if desired.